diff --git a/changelog.d/fix-mt-ehrc-household-member-income.fixed.md b/changelog.d/fix-mt-ehrc-household-member-income.fixed.md
new file mode 100644
index 00000000000..0b6584283d6
--- /dev/null
+++ b/changelog.d/fix-mt-ehrc-household-member-income.fixed.md
@@ -0,0 +1 @@
+Count household members' income in the Montana elderly homeowner/renter credit: a tax unit dependent's gross income, and the income of household members who file their own returns.
diff --git a/changelog.d/fix-mt-separate-filing-allocations.fixed.md b/changelog.d/fix-mt-separate-filing-allocations.fixed.md
new file mode 100644
index 00000000000..c7d3be16f20
--- /dev/null
+++ b/changelog.d/fix-mt-separate-filing-allocations.fixed.md
@@ -0,0 +1 @@
+Montana's federal income tax deduction for spouses filing separately on the same form now attributes federal tax to the spouse who earned the income and caps each spouse at 5,000; the child and dependent care expense deduction phases out on the couple's combined Montana AGI and is halved only for spouses filing separately on the same form; and the elderly homeowner/renter credit counts income without regard to losses.
diff --git a/policyengine_us/parameters/gov/states/mt/tax/income/deductions/itemized/misc_deductions.yaml b/policyengine_us/parameters/gov/states/mt/tax/income/deductions/itemized/misc_deductions.yaml
index 1eeb0351b1a..c4744269768 100644
--- a/policyengine_us/parameters/gov/states/mt/tax/income/deductions/itemized/misc_deductions.yaml
+++ b/policyengine_us/parameters/gov/states/mt/tax/income/deductions/itemized/misc_deductions.yaml
@@ -1,8 +1,11 @@
 description: Montana counts these sources as miscellaneous deductions. 
+# The child and dependent care expense deduction is also a miscellaneous
+# deduction (Form 2 Itemized Deductions Schedule, line 14). It is added
+# separately because its income limit uses the return's own Montana AGI, which
+# differs between a joint return and separate columns (Form 2441-M line 3).
 values:
   2021-01-01:
-    - mt_child_dependent_care_expense_deduction  
-    - casualty_loss_deduction  
+    - casualty_loss_deduction
   2024-01-01: []
     
 metadata:
diff --git a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit.yaml b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit.yaml
index 9e4f2540b71..6f2d13342d2 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit.yaml
@@ -223,3 +223,179 @@
     # Same household, larger exclusion ($12,600) leaves a smaller expected
     # contribution, so the credit is higher: $539.67.
     mt_elderly_homeowner_or_renter_credit: 539.67
+
+- name: 2023 Elderly Homeowner/Renter Credit Schedule worked case, a capital loss does not reduce gross household income
+  period: 2023
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        long_term_capital_gains: -3_000
+        social_security_retirement: 10_000
+        real_estate_taxes: 2_000
+        mt_refundable_credits_before_renter_credit: 0
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_name: MT
+  output:
+    # Line 1 federal AGI 17_000 (Social Security untaxed: 17_000 + 5_000 is
+    # under the 25_000 base); line 5 untaxed Social Security 10_000; line 9
+    # losses 3_000; line 10 gross household income 30_000.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: 30_000
+    # Line 12 = 30_000 - 12_600 = 17_400; line 13 multiplier 0.05;
+    # line 14 = 870.
+    mt_elderly_homeowner_or_renter_credit_net_household_income: 870
+    # Line 18 = 2_000; line 19 = 1_130; line 20 = min(1_130, 1_150);
+    # line 21 = 100% (under 35_000); line 22 = 1_130.
+    mt_elderly_homeowner_or_renter_credit: 1_130
+
+- name: 2024 Schedule 2EC worked case, a dependent's wages count toward gross household income
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        real_estate_taxes: 2_000
+      person2:
+        age: 16
+        employment_income: 10_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # Line 5 pensions 20_000; line 17 other household members' income
+    # 10_000 (the dependent's wages); line 18 gross household income 30_000.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [20_000, 10_000]
+    # Line 19 12_600; line 20 = 30_000 - 12_600 = 17_400; line 21 = 0.05;
+    # line 22 = 870.
+    mt_elderly_homeowner_or_renter_credit_net_household_income: [870, 0]
+    # Line 23 = 2_000; line 26 = 2_000; line 27 = 2_000 - 870 = 1_130;
+    # line 28 = min(1_130, 1_150) = 1_130; line 29 = 100% (line 18 is under
+    # 35_000); line 30 = 1_130. Without the dependent's wages, gross
+    # household income would be 20_000 and the credit the 1_150 cap.
+    mt_elderly_homeowner_or_renter_credit: [1_130, 0]
+
+- name: 2024 Schedule 2EC worked case, a household member filing their own return counts
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        real_estate_taxes: 2_000
+      person2:
+        age: 45
+        taxable_interest_income: 10_000
+    tax_units:
+      tax_unit1:
+        members: [person1]
+      tax_unit2:
+        members: [person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # The claimant's adult daughter lives in the household and files her
+    # own return. § 15-30-2337(7)(a): a household is "an association of
+    # persons who live in the same dwelling"; the 2024 instructions: "You do
+    # not need to be related to be in the same household." Line 5 = 20_000;
+    # line 17 = 10_000 (her interest); line 18 = 30_000; line 22 = 870;
+    # line 27 = 2_000 - 870 = 1_130; line 30 = 1_130. Her tax unit has no
+    # one aged 62 or older, so it claims nothing.
+    mt_elderly_homeowner_or_renter_credit_net_household_income: [870, 0]
+    mt_elderly_homeowner_or_renter_credit: [1_130, 0]
+
+- name: 2024 Schedule 2EC worked case, a dependent's wages move the credit into the 40 percent band
+  period: 2024
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        real_estate_taxes: 2_000
+      person2:
+        age: 17
+        employment_income: 16_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # Line 18 = 20_000 + 16_000 = 36_000; line 20 = 23_400; line 22 =
+    # 23_400 x 0.05 = 1_170; line 27 = 2_000 - 1_170 = 830; line 28 = 830;
+    # line 29 = 40% (36_000 is in the 35_000 to 37_500 band);
+    # line 30 = 830 x 0.4 = 332.
+    mt_elderly_homeowner_or_renter_credit_net_household_income: [1_170, 0]
+    mt_elderly_homeowner_or_renter_credit: [332, 0]
+
+- name: 2024 Schedule 2EC, a dependent's wages bring gross household income to 45,000
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        real_estate_taxes: 2_000
+      person2:
+        age: 17
+        employment_income: 25_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # Line 18 = 45_000. The claimant must have gross household income "less
+    # than $45,000"; line 29 = 0% at 45_000 and above.
+    mt_elderly_homeowner_or_renter_credit: [0, 0]
+
+- name: 2023 Elderly Homeowner/Renter Credit Schedule worked case, a non-filing dependent's wages count
+  period: 2023
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        real_estate_taxes: 2_000
+      person2:
+        age: 16
+        employment_income: 10_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # Line 1 federal AGI 20_000 (the head's return). The dependent's 10_000
+    # of wages is under the 13_850 filing threshold for dependents; the
+    # 2023 instructions say members of the household not filing a Form 2 or
+    # federal Form 1040 "must report all wages" on line 8. Line 10 = 30_000;
+    # line 12 = 17_400; line 13 = 0.05; line 14 = 870; line 15 = 2_000;
+    # line 18 = 2_000; line 19 = 1_130; line 20 = 1_130; line 21 = 100%;
+    # line 22 = 1_130.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [20_000, 10_000]
+    mt_elderly_homeowner_or_renter_credit_net_household_income: [870, 0]
+    mt_elderly_homeowner_or_renter_credit: [1_130, 0]
diff --git a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_gross_household_income.yaml b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_gross_household_income.yaml
index cee870a82af..88dbe3c9d4e 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_gross_household_income.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_gross_household_income.yaml
@@ -37,3 +37,126 @@
     # § 15-30-2337(9)(a)(viii) requires all SS to be counted, even the
     # portion excluded from federal AGI.
     mt_elderly_homeowner_or_renter_credit_gross_household_income: 30_000
+
+- name: A capital loss deducted in federal AGI is added back
+  period: 2023
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        long_term_capital_gains: -3_000
+        mt_refundable_credits_before_renter_credit: 0
+    tax_units:
+      tax_unit:
+        members: [person1]
+    households:
+      household:
+        members: [person1]
+        state_name: MT
+  output:
+    # § 15-30-2337(9)(a): federal AGI "without regard to loss". 2023 Elderly
+    # Homeowner/Renter Credit Schedule:
+    # line 1 federal AGI 20_000 - 3_000 = 17_000; line 9 adds back the 3_000
+    # capital loss; line 10 = 20_000.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: 20_000
+
+- name: A business loss deducted in federal AGI is added back once for the return
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+        mt_refundable_credits_before_renter_credit: 0
+      person2:
+        age: 68
+        self_employment_income: -5_000
+        mt_refundable_credits_before_renter_credit: 0
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # Federal AGI 20_000 - 5_000 = 15_000; the 5_000 loss is added back, so
+    # gross household income is 20_000 (2024 Schedule 2EC line 18). Net household
+    # income: (20_000 - 12_600) * 0.035 = 259, allocated to the head.
+    mt_elderly_homeowner_or_renter_credit_net_household_income: [259, 0]
+
+- name: A dependent's wages count in gross household income
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+        taxable_pension_income: 20_000
+      person2:
+        age: 16
+        employment_income: 10_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # § 15-30-2337(4): "all income received by all individuals of a
+    # household". The dependent's wages are on their own return, not in the
+    # head's federal AGI. 2024 Schedule 2EC: line 5 pensions 20_000 (head);
+    # line 17 income of other household members 10_000 (dependent's wages);
+    # line 18 gross household income 30_000.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [20_000, 10_000]
+
+- name: A dependent's Social Security counts once, in full
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+      person2:
+        age: 17
+        employment_income: 25_000
+        social_security_survivors: 10_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # On the dependent's own return, provisional income is
+    # 25_000 + 10_000 / 2 = 30_000, so 50% x (30_000 - 25_000) = 2_500 of the
+    # benefits is taxable (IRC 86) and is part of dependent_gross_income.
+    dependent_taxable_social_security: [0, 2_500]
+    # § 15-30-2337(9)(a)(viii) counts all Social Security, and 2024
+    # Schedule 2EC line 6 takes "the total taxable and nontaxable" benefits:
+    # wages 25_000 + Social Security 10_000 = 35_000, not 37_500.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [0, 35_000]
+
+- name: A dependent's capital loss does not reduce gross household income
+  period: 2024
+  input:
+    people:
+      person1:
+        age: 70
+      person2:
+        age: 16
+        employment_income: 10_000
+        long_term_capital_gains: -3_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_name: MT
+  output:
+    # § 15-30-2337(9)(a): income "without regard to loss". 2024 Schedule 2EC
+    # line 7: "do not include any capital losses". Wages 10_000 count; the
+    # 3_000 loss does not reduce them.
+    mt_elderly_homeowner_or_renter_credit_gross_household_income: [0, 10_000]
diff --git a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_net_household_income.yaml b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_net_household_income.yaml
index dcc4e975a04..052498f9036 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_net_household_income.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/credits/elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_net_household_income.yaml
@@ -40,3 +40,29 @@
   output:
     # (48,000 - 6,300) x 0.05 with the 2021 exclusion of $6,300.
     mt_elderly_homeowner_or_renter_credit_net_household_income: [2_085, 0]
+
+- name: Gross household income includes members outside the claimant's tax unit
+  period: 2024
+  input:
+    people:
+      person1:
+        mt_elderly_homeowner_or_renter_credit_gross_household_income: 20_000
+        mt_elderly_homeowner_or_renter_credit_eligible: true
+      person2:
+        mt_elderly_homeowner_or_renter_credit_gross_household_income: 10_000
+        mt_elderly_homeowner_or_renter_credit_eligible: false
+    tax_units:
+      tax_unit1:
+        members: [person1]
+      tax_unit2:
+        members: [person2]
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    # 2024 Schedule 2EC line 18 = 20_000 + 10_000 (line 17, the other member)
+    # = 30_000; line 20 = 30_000 - 12_600 = 17_400; line 21 = 0.05;
+    # line 22 = 870, on the claimant's row. person2's tax unit has no
+    # claimant, so 0.
+    mt_elderly_homeowner_or_renter_credit_net_household_income: [870, 0]
diff --git a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/cdcc/mt_child_dependent_care_expense_deduction.yaml b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/cdcc/mt_child_dependent_care_expense_deduction.yaml
index df49679c68b..4cdfcd2395d 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/cdcc/mt_child_dependent_care_expense_deduction.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/cdcc/mt_child_dependent_care_expense_deduction.yaml
@@ -19,8 +19,8 @@
         state_code: MT
   output:
     # One qualifying individual caps expenses at 2_400.
-    # Head receives half: 2_400 * 0.5 = 1_200.
-    mt_child_dependent_care_expense_deduction: [1_200, 0]
+    # A single filer enters the full line 7 amount: 2_400.
+    mt_child_dependent_care_expense_deduction: [2_400, 0]
 
 - name: Care expense greater than maximum amount with moderate AGI and 1 child
   period: 2021
@@ -43,8 +43,8 @@
         state_code: MT
   output:
     # 2_400 - 0.5 * (20_000 - 18_000) = 1_400.
-    # Head receives half: 1_400 * 0.5 = 700.
-    mt_child_dependent_care_expense_deduction: [700, 0]
+    # A single filer enters the full line 7 amount: 1_400.
+    mt_child_dependent_care_expense_deduction: [1_400, 0]
 
 - name: Care expense greater than maximum amount with high AGI and 1 child
   period: 2021
@@ -92,8 +92,8 @@
         state_code: MT
   output:
     # Two qualifying individuals cap expenses at 3_600.
-    # Head receives half: 3_600 * 0.5 = 1_800.
-    mt_child_dependent_care_expense_deduction: [1_800, 0, 0]
+    # A single filer enters the full line 7 amount: 3_600.
+    mt_child_dependent_care_expense_deduction: [3_600, 0, 0]
 
 - name: Care expense greater than maximum amount with moderate AGI and 2 children
   period: 2021
@@ -119,8 +119,8 @@
         state_code: MT
   output:
     # 3_600 - 0.5 * (22_000 - 18_000) = 1_600.
-    # Head receives half: 1_600 * 0.5 = 800.
-    mt_child_dependent_care_expense_deduction: [800, 0, 0]
+    # A single filer enters the full line 7 amount: 1_600.
+    mt_child_dependent_care_expense_deduction: [1_600, 0, 0]
 
 - name: Care expense greater than maximum amount with high AGI and 2 children
   period: 2021
@@ -174,8 +174,8 @@
         state_code: MT
   output:
     # Three qualifying individuals cap expenses at 4_800.
-    # Head receives half: 4_800 * 0.5 = 2_400.
-    mt_child_dependent_care_expense_deduction: [2_400, 0, 0, 0]
+    # A single filer enters the full line 7 amount: 4_800.
+    mt_child_dependent_care_expense_deduction: [4_800, 0, 0, 0]
 
 - name: Care expense greater than maximum amount with moderate AGI and 3 children
   period: 2021
@@ -204,8 +204,8 @@
         state_code: MT
   output:
     # 4_800 - 0.5 * (24_000 - 18_000) = 1_800.
-    # Head receives half: 1_800 * 0.5 = 900.
-    mt_child_dependent_care_expense_deduction: [900, 0, 0, 0]
+    # A single filer enters the full line 7 amount: 1_800.
+    mt_child_dependent_care_expense_deduction: [1_800, 0, 0, 0]
 
 - name: Care expense greater than maximum amount with high AGI and 3 children
   period: 2021
@@ -255,8 +255,8 @@
         members: [person1, person2]
         state_code: MT
   output:
-    # Head receives half: 2_000 * 0.5 = 1_000.
-    mt_child_dependent_care_expense_deduction: [1_000, 0]
+    # A single filer enters the full line 7 amount: 2_000.
+    mt_child_dependent_care_expense_deduction: [2_000, 0]
 
 - name: Care expense less than maximum amount with high AGI and 1 child
   period: 2021
@@ -303,8 +303,8 @@
         members: [person1, person2, person3]
         state_code: MT
   output:
-    # Head receives half: 3_000 * 0.5 = 1_500.
-    mt_child_dependent_care_expense_deduction: [1_500, 0, 0]
+    # A single filer enters the full line 7 amount: 3_000.
+    mt_child_dependent_care_expense_deduction: [3_000, 0, 0]
 
 - name: Care expense less than maximum amount with high AGI and 2 children
   period: 2021
@@ -357,8 +357,8 @@
         members: [person1, person2, person3, person4]
         state_code: MT
   output:
-    # Head receives half: 4_000 * 0.5 = 2_000.
-    mt_child_dependent_care_expense_deduction: [2_000, 0, 0, 0]
+    # A single filer enters the full line 7 amount: 4_000.
+    mt_child_dependent_care_expense_deduction: [4_000, 0, 0, 0]
 
 - name: Care expense less than maximum amount with high AGI and 3 children
   period: 2021
@@ -441,8 +441,8 @@
         state_code: MT
   output:
     # One qualifying individual caps expenses at 2_400 (Form 2441-M line 2).
-    # Head receives half: 2_400 * 0.5 = 1_200.
-    mt_child_dependent_care_expense_deduction: [1_200, 0]
+    # A single filer enters the full line 7 amount: 2_400.
+    mt_child_dependent_care_expense_deduction: [2_400, 0]
 
 - name: Case 18, capable disabled adult care expenses are excluded.
   period: 2021
@@ -470,8 +470,8 @@
         state_code: MT
   output:
     # Only the child qualifies, so 2_000 of adult care expenses are excluded.
-    # Head receives half of the 1_000 childcare expense deduction.
-    mt_child_dependent_care_expense_deduction: [500, 0, 0]
+    # A single filer enters the full 1_000 childcare expense deduction.
+    mt_child_dependent_care_expense_deduction: [1_000, 0, 0]
 
 - name: Case 19, a child and an incapacitated adult dependent both count toward the tier.
   period: 2021
@@ -500,8 +500,8 @@
   output:
     # Two qualifying individuals cap expenses at 3_600 (Form 2441-M line 2).
     # Childcare 3_000 + adult care 1_000 = 4_000, capped at 3_600.
-    # Head receives half: 3_600 * 0.5 = 1_800.
-    mt_child_dependent_care_expense_deduction: [1_800, 0, 0]
+    # A single filer enters the full line 7 amount: 3_600.
+    mt_child_dependent_care_expense_deduction: [3_600, 0, 0]
 
 - name: Case 20, adult care expenses cannot exceed the one-qualifying-individual tier cap.
   period: 2021
@@ -525,8 +525,8 @@
         state_code: MT
   output:
     # One qualifying individual caps expenses at 2_400 (Form 2441-M line 2).
-    # Head receives half: 2_400 * 0.5 = 1_200.
-    mt_child_dependent_care_expense_deduction: [1_200, 0]
+    # A single filer enters the full line 7 amount: 2_400.
+    mt_child_dependent_care_expense_deduction: [2_400, 0]
 
 - name: Case 21, incapacitated spouse care expenses qualify.
   period: 2021
@@ -606,8 +606,8 @@
   output:
     # One qualifying individual caps expenses at 2_400 (Form 2441-M line 2).
     # Childcare 1_500 + care expenses 1_000 = 2_500, capped at 2_400.
-    # Head receives half: 2_400 * 0.5 = 1_200.
-    mt_child_dependent_care_expense_deduction: [1_200, 0]
+    # A single filer enters the full line 7 amount: 2_400.
+    mt_child_dependent_care_expense_deduction: [2_400, 0]
 
 - name: Case 24, combined childcare and adult care expenses are capped by the three-qualifying-individual tier.
   period: 2021
@@ -643,3 +643,109 @@
     # Childcare 4_000 + adult care 2_000 = 6_000, capped at 4_800.
     # The deduction is divided equally between spouses: 4_800 * 0.5 = 2_400.
     mt_child_dependent_care_expense_deduction: [2_400, 2_400, 0, 0]
+
+- name: Case 25, the phase-out uses the couple's combined Montana AGI, then each column gets half
+  period: 2022
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        is_tax_unit_head: true
+        mt_agi_indiv: 20_000
+      person2:
+        is_tax_unit_spouse: true
+        mt_agi_indiv: 0
+      person3:
+        age: 5
+        is_tax_unit_dependent: true
+    tax_units:
+      tax_unit:
+        members: [person1, person2, person3]
+        tax_unit_childcare_expenses: 3_000
+    households:
+      household:
+        members: [person1, person2, person3]
+        state_code: MT
+  output:
+    # Form 2441-M (2022): line 2 = min(3_000, 2_400) = 2_400; line 3 adds
+    # columns A and B of Form 2 line 14 = 20_000 + 0 = 20_000; line 5 = 2_000;
+    # line 6 = 1_000; line 7 = 1_400. Filing separately on the same form, each
+    # spouse enters one-half of line 7 (700). Applying the phase-out to each
+    # spouse's own AGI would give 700 and 1_200.
+    mt_child_dependent_care_expense_deduction_eligible_expenses: 2_400
+    mt_child_dependent_care_expense_deduction: [700, 700, 0]
+
+- name: Case 26, a joint return uses the return's Montana AGI and claims the whole deduction
+  period: 2022
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        is_tax_unit_head: true
+      person2:
+        is_tax_unit_spouse: true
+      person3:
+        age: 5
+        is_tax_unit_dependent: true
+    tax_units:
+      tax_unit:
+        members: [person1, person2, person3]
+        tax_unit_childcare_expenses: 3_000
+        mt_agi_joint: 20_000
+    households:
+      household:
+        members: [person1, person2, person3]
+        state_code: MT
+  output:
+    # Form 2441-M lines 2-7 with line 3 = 20_000: 2_400 - 0.5 * 2_000 = 1_400,
+    # entered in full on the joint return.
+    mt_child_dependent_care_expense_deduction_joint: 1_400
+
+- name: Case 27, the joint deduction is repealed beginning in 2024
+  period: 2024
+  input:
+    people:
+      person1:
+        is_tax_unit_head: true
+      person2:
+        age: 5
+        is_tax_unit_dependent: true
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+        tax_unit_childcare_expenses: 3_000
+        mt_agi_joint: 10_000
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    mt_child_dependent_care_expense_deduction_joint: 0
+
+- name: Case 28, a married person filing on a separate form cannot take the deduction
+  period: 2022
+  input:
+    people:
+      person1:
+        is_tax_unit_head: true
+        mt_agi_indiv: 15_000
+      person2:
+        age: 5
+        is_tax_unit_dependent: true
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+        filing_status: SEPARATE
+        tax_unit_childcare_expenses: 2_000
+        mt_agi_joint: 15_000
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    # MCA 15-30-2131(1)(c)(vi)(A): "married couples shall file a joint return
+    # or file separately on the same form". 2022 instructions, line 14: "If
+    # you are married and filing separately on separate forms, or your spouse
+    # is not filing, you cannot qualify for this deduction."
+    mt_child_dependent_care_expense_deduction: [0, 0]
+    mt_child_dependent_care_expense_deduction_joint: 0
diff --git a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_federal_income_tax_deduction_for_federal_itemization_indiv.yaml b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_federal_income_tax_deduction_for_federal_itemization_indiv.yaml
index e0fed58cbeb..941a60e6c1b 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_federal_income_tax_deduction_for_federal_itemization_indiv.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_federal_income_tax_deduction_for_federal_itemization_indiv.yaml
@@ -1,39 +1,46 @@
-- name: Deductions split evenly 
+# 2022 Form 2 instructions, Itemized Deductions Schedule, line 4 (PDF p. 33):
+# withholding is reported by the spouse who earned the income, and each
+# spouse filing separately is limited to 5,000.
+
+- name: Each spouse deducts their own withholding estimate
   period: 2021
   input:
     people:
       person1:
-        mt_agi_indiv: 500
+        mt_withheld_income_tax: 3_000
+        adjusted_gross_income_person: 50_000
       person2:
-        mt_agi_indiv: 500
+        mt_withheld_income_tax: 1_000
+        adjusted_gross_income_person: 50_000
     tax_units:
       tax_unit:
         members: [person1, person2]
-        mt_federal_income_tax_deduction_for_federal_itemization: 3_000
         state_filing_status_if_married_filing_separately_on_same_return: SEPARATE
     households:
       household:
         members: [person1, person2]
         state_code: MT
   output:
-    mt_federal_income_tax_deduction_for_federal_itemization_indiv: [1_500, 1_500]
+    # Prorating the couple's 4_000 by Montana AGI would give 2_000 / 2_000.
+    mt_federal_income_tax_deduction_for_federal_itemization_indiv: [3_000, 1_000]
 
-- name: Deductions capped at max amount individually
+- name: Each spouse is capped at 5,000 on their own amount
   period: 2021
   input:
     people:
       person1:
-        mt_agi_indiv: 100
+        mt_withheld_income_tax: 7_000
+        adjusted_gross_income_person: 60_000
       person2:
-        mt_agi_indiv: 900
+        mt_withheld_income_tax: 1_000
+        adjusted_gross_income_person: 40_000
     tax_units:
       tax_unit:
         members: [person1, person2]
-        mt_federal_income_tax_deduction_for_federal_itemization: 10_000
         state_filing_status_if_married_filing_separately_on_same_return: SEPARATE
     households:
       household:
         members: [person1, person2]
         state_code: MT
   output:
-    mt_federal_income_tax_deduction_for_federal_itemization_indiv: [1_000, 5_000]
+    mt_federal_income_tax_deduction_for_federal_itemization_indiv: [5_000, 1_000]
diff --git a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_joint.yaml b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_joint.yaml
index 8a1114f4ef5..41a8d808975 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_joint.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_joint.yaml
@@ -7,7 +7,7 @@
      mt_salt_deduction: 5_000
      mt_medical_expense_deduction_joint: 1_000
      casualty_loss_deduction: 400
-     mt_child_dependent_care_expense_deduction: 123
+     mt_child_dependent_care_expense_deduction_joint: 123
      mt_federal_income_tax_deduction_for_federal_itemization: 2_000
      state_code: MT
   output:
@@ -22,7 +22,7 @@
      mt_salt_deduction: 10_000
      mt_medical_expense_deduction_joint: 3_000 
      casualty_loss_deduction: 0
-     mt_child_dependent_care_expense_deduction: 0
+     mt_child_dependent_care_expense_deduction_joint: 0
      mt_federal_income_tax_deduction_for_federal_itemization: 2_000
      state_code: MT
   output:
@@ -37,7 +37,7 @@
      mt_salt_deduction: 1_035
      mt_medical_expense_deduction_joint: 3_000
      casualty_loss_deduction: 745
-     mt_child_dependent_care_expense_deduction: 1_004
+     mt_child_dependent_care_expense_deduction_joint: 1_004
      mt_federal_income_tax_deduction_for_federal_itemization: 2_000
      state_code: MT
   output:
diff --git a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_federal_income_tax_deduction_indiv.yaml b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_federal_income_tax_deduction_indiv.yaml
index 4b0d588ce5e..c7604752c4d 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_federal_income_tax_deduction_indiv.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_federal_income_tax_deduction_indiv.yaml
@@ -1,39 +1,120 @@
-- name: Deductions split evenly 
+# 2022 Form 2 instructions, Itemized Deductions Schedule, line 4 (PDF p. 33):
+# "If you are married filing separately with your spouse, the federal income
+# tax withheld should be reported by the spouse who earned the income." and
+# "If your filing status is Married Filing Separately, you are each limited to
+# the $5,000 federal tax deduction and you cannot arbitrarily allocate this
+# deduction between spouses."
+
+- name: Federal income tax is attributed by each spouse's share of federal AGI
   period: 2021
+  absolute_error_margin: 0.01
   input:
     people:
       person1:
-        mt_agi_indiv: 500
+        adjusted_gross_income_person: 60_000
       person2:
-        mt_agi_indiv: 500
+        adjusted_gross_income_person: 40_000
     tax_units:
       tax_unit:
         members: [person1, person2]
-        mt_federal_income_tax_deduction_unit: 3_000
+        income_tax_before_refundable_credits: 3_000
         state_filing_status_if_married_filing_separately_on_same_return: SEPARATE
     households:
       household:
         members: [person1, person2]
         state_code: MT
   output:
-    mt_federal_income_tax_deduction_indiv: [1_500, 1_500]
+    mt_federal_income_tax_deduction_share: [0.6, 0.4]
+    mt_federal_income_tax_deduction_indiv: [1_800, 1_200]
 
-- name: Deductions capped at max amount individually
-  period: 2021
+- name: Each spouse's own federal tax is capped at 5,000, not a share of the joint cap
+  period: 2022
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        adjusted_gross_income_person: 80_000
+      person2:
+        adjusted_gross_income_person: 20_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+        income_tax_before_refundable_credits: 30_000
+        state_filing_status_if_married_filing_separately_on_same_return: SEPARATE
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    # Federal tax attributed 24_000 / 6_000; each spouse is limited to 5_000.
+    # Capping the couple at 10_000 first and then splitting 80% / 20% would
+    # give 5_000 / 2_000.
+    mt_federal_income_tax_deduction_indiv: [5_000, 5_000]
+
+- name: Montana-only subtractions do not shift federal tax between spouses
+  period: 2022
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        adjusted_gross_income_person: 50_000
+      person2:
+        adjusted_gross_income_person: 50_000
+        mt_subtractions: 20_000
+    tax_units:
+      tax_unit:
+        members: [person1, person2]
+        income_tax_before_refundable_credits: 6_000
+        state_filing_status_if_married_filing_separately_on_same_return: SEPARATE
+    households:
+      household:
+        members: [person1, person2]
+        state_code: MT
+  output:
+    # The federal tax was paid on 50_000 of federal income each. Prorating by
+    # Montana AGI (50_000 / 30_000) would give 3_750 / 2_250.
+    mt_federal_income_tax_deduction_indiv: [3_000, 3_000]
+
+- name: A dependent's income does not draw a share of the parents' federal tax
+  period: 2022
+  absolute_error_margin: 0.01
+  input:
+    people:
+      person1:
+        adjusted_gross_income_person: 40_000
+      person2:
+        adjusted_gross_income_person: 0
+      person3:
+        is_tax_unit_dependent: true
+        adjusted_gross_income_person: 0
+    tax_units:
+      tax_unit:
+        members: [person1, person2, person3]
+        income_tax_before_refundable_credits: 2_000
+        state_filing_status_if_married_filing_separately_on_same_return: SEPARATE
+    households:
+      household:
+        members: [person1, person2, person3]
+        state_code: MT
+  output:
+    mt_federal_income_tax_deduction_indiv: [2_000, 0, 0]
+
+- name: The deduction is repealed from 2024
+  period: 2024
   input:
     people:
       person1:
-        mt_agi_indiv: 100
+        adjusted_gross_income_person: 60_000
       person2:
-        mt_agi_indiv: 900
+        adjusted_gross_income_person: 40_000
     tax_units:
       tax_unit:
         members: [person1, person2]
-        mt_federal_income_tax_deduction_unit: 10_000
+        income_tax_before_refundable_credits: 3_000
         state_filing_status_if_married_filing_separately_on_same_return: SEPARATE
     households:
       household:
         members: [person1, person2]
         state_code: MT
   output:
-    mt_federal_income_tax_deduction_indiv: [1_000, 5_000]
+    mt_federal_income_tax_deduction_indiv: [0, 0]
diff --git a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_itemized_deductions_joint.yaml b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_itemized_deductions_joint.yaml
index 6b25795e07a..329641eafa3 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_itemized_deductions_joint.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/deductions/itemized/mt_itemized_deductions_joint.yaml
@@ -7,7 +7,7 @@
      mt_salt_deduction: 5_000
      mt_medical_expense_deduction_joint: 1_000
      casualty_loss_deduction: 400
-     mt_child_dependent_care_expense_deduction: 123
+     mt_child_dependent_care_expense_deduction_joint: 123
      mt_federal_income_tax_deduction_unit: 2_000
      state_code: MT
   output:
@@ -22,7 +22,7 @@
      mt_salt_deduction: 10_000
      mt_medical_expense_deduction_joint: 3_000 
      casualty_loss_deduction: 0
-     mt_child_dependent_care_expense_deduction: 0
+     mt_child_dependent_care_expense_deduction_joint: 0
      mt_federal_income_tax_deduction_unit: 2_000
      state_code: MT
   output:
@@ -37,7 +37,7 @@
      mt_salt_deduction: 1_035
      mt_medical_expense_deduction_joint: 3_000
      casualty_loss_deduction: 745
-     mt_child_dependent_care_expense_deduction: 1_004
+     mt_child_dependent_care_expense_deduction_joint: 1_004
      mt_federal_income_tax_deduction_unit: 2_000
      state_code: MT
   output:
diff --git a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/integration.yaml b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/integration.yaml
index 1427235ceaa..687a3ecabbf 100644
--- a/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/integration.yaml
+++ b/policyengine_us/tests/policy/baseline/gov/states/mt/tax/income/integration.yaml
@@ -564,9 +564,12 @@
   output:
     # $2,500 per-return cap split $1,250 per spouse (was double-counted to $5,000)
     mt_income_tax_rebate: [1_250, 1_250]
-    # Files separately: 5,099.51 - 1,250 + 3,719.51 - 1,250 = 6,319.02
+    # Each spouse deducts their own federal income tax up to $5,000 (2021
+    # federal tax 21,175 attributed 90/160 and 70/160), so both itemize 5,000
+    # over the 4,830 standard deduction. Taxable income 82,420 and 62,420.
+    # Files separately: 5,087.78 - 1,250 + 3,707.78 - 1,250 = 6,295.56
     # (old double-count subtracted $2,500 per column, understating tax by $2,500)
-    mt_income_tax: 6_319.02
+    mt_income_tax: 6_295.56
 
 - name: MT property tax rebate is not a Form 2 credit and is not tripled (issue 9347)
   absolute_error_margin: 0.01
diff --git a/policyengine_us/tests/test_mt_elderly_credit_household_members.py b/policyengine_us/tests/test_mt_elderly_credit_household_members.py
new file mode 100644
index 00000000000..d1c1fff15ef
--- /dev/null
+++ b/policyengine_us/tests/test_mt_elderly_credit_household_members.py
@@ -0,0 +1,241 @@
+"""Property tests: household members' income in the Montana elderly
+homeowner/renter credit.
+
+MCA 15-30-2337(4) defines gross household income as "all income received by
+all individuals of a household while they are members of the household", and
+(9)(a) counts federal adjusted gross income "without regard to loss" plus
+nontaxable income, including all Social Security. The 2024 Schedule 2EC lists
+the claimant's income by category (line 7: "do not include any capital
+losses"; line 6: "the total taxable and nontaxable Social Security benefits")
+and puts the income of "other members of your household" on line 17.
+
+The model reaches the same quantity by two paths. A member who is the
+claimant's tax unit dependent is counted through dependent_gross_income and
+dependent_taxable_social_security; a member who files their own return is
+counted through federal AGI, the loss add-back and taxable_social_security.
+For a claimant aged 62 or older (taxable pension, Social Security, property
+tax, rent) and one other adult in the household (taxable interest, a
+long-term capital gain or loss, Social Security):
+
+1. The household's gross household income is the claimant's pension and
+   Social Security plus the other member's interest, positive capital gain
+   and Social Security, whichever path counts the other member.
+2. So the claimant's credit is the same in both arrangements, and the other
+   member's own tax unit, with no one aged 62 or older, claims nothing.
+3. The credit is between zero and the cap, and zero when gross household
+   income is $45,000 or more (line 29).
+4. More interest for the other member never raises the credit: gross
+   household income rises, so net household income (line 22) cannot fall
+   and the credit multiplier (line 29) cannot rise.
+
+No one has earned income, so no refundable credit enters line 8. The model
+computes in single precision, so comparisons allow one cent or eight float32
+spacings at the household's total income, whichever is larger.
+"""
+
+import numpy as np
+import pytest
+
+from policyengine_us import Simulation
+
+YEAR = 2024
+CAP = 1_150
+INCOME_LIMIT = 45_000
+TOLERANCE = 0.01
+
+
+def build_situation(cases):
+    """Three Montana households per case: (A) the other member is the
+    claimant's dependent, (B) the other member files their own return and
+    (C) as A with the other member's extra interest added."""
+    people, tax_units, marital_units, spm_units, families, households = (
+        {},
+        {},
+        {},
+        {},
+        {},
+        {},
+    )
+    for i, c in enumerate(cases):
+        for arrangement in "ABC":
+            key = f"{arrangement}{i}"
+            claimant, member = f"claimant_{key}", f"member_{key}"
+            interest = c["interest"] + (
+                c["extra_interest"] if arrangement == "C" else 0
+            )
+            people[claimant] = {
+                "age": {YEAR: c["claimant_age"]},
+                "taxable_pension_income": {YEAR: c["pension"]},
+                "social_security_retirement": {YEAR: c["claimant_ss"]},
+                "real_estate_taxes": {YEAR: c["property_tax"]},
+                "rent": {YEAR: c["rent"]},
+            }
+            people[member] = {
+                "age": {YEAR: c["member_age"]},
+                "taxable_interest_income": {YEAR: interest},
+                "long_term_capital_gains": {YEAR: c["capital_gain"]},
+                "social_security_survivors": {YEAR: c["member_ss"]},
+            }
+            if arrangement == "B":
+                tax_units[f"tax_unit_{key}"] = {"members": [claimant]}
+                tax_units[f"tax_unit_{key}_member"] = {"members": [member]}
+            else:
+                tax_units[f"tax_unit_{key}"] = {"members": [claimant, member]}
+            # Set for everyone: an input for some people gives the others
+            # its default (false), not its formula. Without it the model
+            # takes an adult in the claimant's tax unit to be the spouse.
+            people[claimant]["is_tax_unit_dependent"] = {YEAR: False}
+            people[member]["is_tax_unit_dependent"] = {YEAR: arrangement != "B"}
+            marital_units[f"marital_unit_{key}"] = {"members": [claimant]}
+            marital_units[f"marital_unit_{key}_member"] = {"members": [member]}
+            spm_units[f"spm_unit_{key}"] = {"members": [claimant, member]}
+            families[f"family_{key}"] = {"members": [claimant, member]}
+            households[f"household_{key}"] = {
+                "members": [claimant, member],
+                "state_code": {YEAR: "MT"},
+            }
+    return {
+        "people": people,
+        "tax_units": tax_units,
+        "marital_units": marital_units,
+        "spm_units": spm_units,
+        "families": families,
+        "households": households,
+    }
+
+
+def calculate(cases):
+    """Household totals of gross household income and the credit, and the
+    other member's credit, as arrays indexed [arrangement, case]."""
+    simulation = Simulation(situation=build_situation(cases))
+    gross_household_income = np.asarray(
+        simulation.calculate(
+            "mt_elderly_homeowner_or_renter_credit_gross_household_income",
+            YEAR,
+            map_to="household",
+        )
+    )
+    credit = np.asarray(
+        simulation.calculate(
+            "mt_elderly_homeowner_or_renter_credit", YEAR, map_to="household"
+        )
+    )
+    person_credit = np.asarray(
+        simulation.calculate("mt_elderly_homeowner_or_renter_credit", YEAR)
+    )
+    dependent = np.asarray(simulation.calculate("is_tax_unit_dependent", YEAR))
+    # Households and people are in situation order: A0, B0, C0, A1, ...;
+    # the member follows the claimant.
+    n = len(cases)
+    return {
+        "gross_household_income": gross_household_income.reshape(n, 3).T,
+        "credit": credit.reshape(n, 3).T,
+        "member_credit": person_credit[1::2].reshape(n, 3).T,
+        "member_is_dependent": dependent[1::2].reshape(n, 3).T,
+    }
+
+
+def expected_gross_household_income(c, extra_interest=0):
+    """2024 Schedule 2EC line 18: lines 5 and 6 for the claimant, and line
+    17 for the other member's interest, gains and Social Security."""
+    claimant = c["pension"] + c["claimant_ss"]
+    member = c["interest"] + extra_interest + max(0, c["capital_gain"])
+    return claimant + member + c["member_ss"]
+
+
+def tolerance(amount):
+    return max(TOLERANCE, 8 * float(np.spacing(np.float32(abs(amount)))))
+
+
+def assert_properties(cases):
+    result = calculate(cases)
+    a, b, c_ = 0, 1, 2
+    # The other member is a dependent in A and C and their own filer in B.
+    assert result["member_is_dependent"].tolist() == [
+        [True] * len(cases),
+        [False] * len(cases),
+        [True] * len(cases),
+    ]
+    for i, c in enumerate(cases):
+        expected = expected_gross_household_income(c)
+        tol = tolerance(expected + c["extra_interest"])
+        ghi = result["gross_household_income"]
+        credit = result["credit"]
+        # 1. Both paths give the Schedule 2EC total.
+        assert ghi[a, i] == pytest.approx(expected, abs=tol), c
+        assert ghi[b, i] == pytest.approx(expected, abs=tol), c
+        assert ghi[c_, i] == pytest.approx(expected + c["extra_interest"], abs=tol), c
+        # 2. The claimant's credit does not depend on the arrangement, and
+        # the other member claims nothing.
+        assert credit[a, i] == pytest.approx(credit[b, i], abs=tol), c
+        assert np.all(result["member_credit"][:, i] == 0), c
+        for arrangement in (a, b, c_):
+            # 3. Bounds and the income limit.
+            assert -tol <= credit[arrangement, i] <= CAP + tol, c
+            if ghi[arrangement, i] >= INCOME_LIMIT + tol:
+                assert credit[arrangement, i] == 0, c
+        # 4. More income never raises the credit.
+        assert credit[c_, i] <= credit[a, i] + tol, c
+
+
+def test_worked_example():
+    """2024 Schedule 2EC: line 5 pension 20,000; line 17 the other member's
+    interest 10,000; line 18 = 30,000; line 22 = (30,000 - 12,600) x 0.05 =
+    870; line 27 = 2,000 - 870 = 1,130; line 30 = 1,130 in both
+    arrangements. With 5,000 more interest, line 18 = 35,000; line 22 =
+    1,120; line 28 = 880; line 29 = 40%; line 30 = 352."""
+    case = {
+        "claimant_age": 70,
+        "pension": 20_000,
+        "claimant_ss": 0,
+        "property_tax": 2_000,
+        "rent": 0,
+        "member_age": 40,
+        "interest": 10_000,
+        "capital_gain": 0,
+        "member_ss": 0,
+        "extra_interest": 5_000,
+    }
+    result = calculate([case])
+    assert result["gross_household_income"][:, 0] == pytest.approx(
+        [30_000, 30_000, 35_000], abs=TOLERANCE
+    )
+    assert result["credit"][:, 0] == pytest.approx([1_130, 1_130, 352], abs=TOLERANCE)
+    assert_properties([case])
+
+
+try:
+    import hypothesis
+    import hypothesis.strategies as st
+except ImportError:  # pragma: no cover
+    hypothesis = None
+
+if hypothesis is not None:
+    # Each example builds a Simulation; on a loaded runner input generation
+    # can trip the too_slow health check, which says nothing about the model.
+    SLOW = [hypothesis.HealthCheck.too_slow]
+
+    def amount(high):
+        return st.one_of(st.just(0), st.integers(min_value=1, max_value=high))
+
+    @st.composite
+    def case(draw):
+        return {
+            "claimant_age": draw(st.integers(min_value=62, max_value=90)),
+            "pension": draw(amount(40_000)),
+            "claimant_ss": draw(amount(30_000)),
+            "property_tax": draw(st.integers(min_value=0, max_value=4_000)),
+            "rent": draw(amount(12_000)),
+            "member_age": draw(st.integers(min_value=15, max_value=61)),
+            "interest": draw(amount(40_000)),
+            "capital_gain": draw(
+                st.one_of(st.just(0), st.integers(min_value=-20_000, max_value=40_000))
+            ),
+            "member_ss": draw(amount(20_000)),
+            "extra_interest": draw(st.integers(min_value=1, max_value=20_000)),
+        }
+
+    @hypothesis.settings(max_examples=15, deadline=None, suppress_health_check=SLOW)
+    @hypothesis.given(st.lists(case(), min_size=1, max_size=8))
+    def test_properties(cases):
+        assert_properties(cases)
diff --git a/policyengine_us/variables/gov/states/mt/tax/income/credits/mt_elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit.py b/policyengine_us/variables/gov/states/mt/tax/income/credits/mt_elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit.py
index d233f4ad92c..f2cbf8e8162 100644
--- a/policyengine_us/variables/gov/states/mt/tax/income/credits/mt_elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit.py
+++ b/policyengine_us/variables/gov/states/mt/tax/income/credits/mt_elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit.py
@@ -8,16 +8,23 @@ class mt_elderly_homeowner_or_renter_credit(Variable):
     unit = USD
     definition_period = YEAR
     defined_for = "mt_elderly_homeowner_or_renter_credit_eligible"
+    reference = (
+        "https://mca.legmt.gov/bills/mca/title_0150/chapter_0300/part_0230/section_0370/0150-0300-0230-0370.html",
+        # 2024 Form 2, Schedule 2EC
+        "https://revenue.mt.gov/files/forms/Montana-Individual-Income-Tax-Return-Form-2/2024_Montana_Individual_Income_Tax_Return_Form_2.pdf#page=10",
+    )
 
     def formula(person, period, parameters):
         p = parameters(
             period
         ).gov.states.mt.tax.income.credits.elderly_homeowner_or_renter
-        # § 15-30-2337(4) defines gross household income at the household
-        # (tax-unit) level, so the multiplier phase-out must be applied to
-        # the aggregated amount rather than each person's share.
+        # § 15-30-2337(4) defines gross household income as "all income
+        # received by all individuals of a household", including members
+        # outside the claimant's return (2024 Schedule 2EC line 17), so the
+        # multiplier phase-out applies to the household's total income rather
+        # than each person's share.
         gross_household_income = add(
-            person.tax_unit,
+            person.household,
             period,
             ["mt_elderly_homeowner_or_renter_credit_gross_household_income"],
         )
diff --git a/policyengine_us/variables/gov/states/mt/tax/income/credits/mt_elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_gross_household_income.py b/policyengine_us/variables/gov/states/mt/tax/income/credits/mt_elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_gross_household_income.py
index e4a6976f246..735e9a7bafc 100644
--- a/policyengine_us/variables/gov/states/mt/tax/income/credits/mt_elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_gross_household_income.py
+++ b/policyengine_us/variables/gov/states/mt/tax/income/credits/mt_elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_gross_household_income.py
@@ -8,16 +8,49 @@ class mt_elderly_homeowner_or_renter_credit_gross_household_income(Variable):
     unit = USD
     definition_period = YEAR
     defined_for = StateCode.MT
-    reference = "https://law.justia.com/codes/montana/2022/title-15/chapter-30/part-23/section-15-30-2337/"
+    reference = (
+        "https://law.justia.com/codes/montana/2022/title-15/chapter-30/part-23/section-15-30-2337/",
+        "https://mca.legmt.gov/bills/mca/title_0150/chapter_0300/part_0230/section_0370/0150-0300-0230-0370.html",
+        # 2023 Form 2 instructions, Elderly Homeowner/Renter Credit Schedule, line 9
+        # (renamed Schedule 2EC from 2024)
+        "https://revenue.mt.gov/files/forms/Montana-Individual-Income-Tax-Return-Form-2-Instructions/2023_Montana_Individual_Income_Tax_Return_Form_2_Instructions.pdf#page=52",
+        # 2023 instructions, lines 1 and 8: the returns of each member of
+        # the household, and the wages of members who do not file
+        "https://revenue.mt.gov/files/forms/Montana-Individual-Income-Tax-Return-Form-2-Instructions/2023_Montana_Individual_Income_Tax_Return_Form_2_Instructions.pdf#page=51",
+        # 2024 Schedule 2EC, line 17: income received by other members of
+        # the household
+        "https://revenue.mt.gov/files/forms/Montana-Individual-Income-Tax-Return-Form-2-Instructions/2024_Montana_Individual_Income_Tax_Return_Form_2_Instructions.pdf#page=48",
+    )
 
     def formula(person, period, parameters):
         p = parameters(
             period
         ).gov.states.mt.tax.income.credits.elderly_homeowner_or_renter
         sources = add(person, period, p.gross_income_sources)
-        # AGI only captures the taxable portion of Social Security; add the
-        # untaxed portion so all SS is counted per § 15-30-2337(9)(a)(viii).
+        # Gross household income counts every member of the household
+        # (§ 15-30-2337(4)). A tax unit dependent's income is on their own
+        # return, not in adjusted_gross_income_person (irs_gross_income
+        # leaves dependents out), so count their gross income directly:
+        # gains but not losses, and the taxable part of their Social
+        # Security.
+        is_dependent = person("is_tax_unit_dependent", period)
+        dependent_income = is_dependent * person("dependent_gross_income", period)
+        # The income above counts only the taxable portion of Social
+        # Security: a filer's in federal AGI, a dependent's in their own
+        # gross income. Add the untaxed portion so all SS is counted per
+        # § 15-30-2337(9)(a)(viii).
         social_security = person("social_security", period)
-        taxable_social_security = person("taxable_social_security", period)
+        taxable_social_security = add(
+            person,
+            period,
+            ["taxable_social_security", "dependent_taxable_social_security"],
+        )
         untaxed_social_security = max_(social_security - taxable_social_security, 0)
-        return sources + untaxed_social_security
+        # Income is federal AGI "without regard to loss" (§ 15-30-2337(9)(a)),
+        # so the business and capital losses deducted in computing federal
+        # AGI are added back (2023 Elderly Homeowner/Renter Credit Schedule,
+        # line 9: "The gross household income cannot be reduced by any
+        # losses."). The return's losses are added once, to the head.
+        head = person("is_tax_unit_head", period)
+        losses = head * person.tax_unit("loss_ald", period)
+        return sources + dependent_income + untaxed_social_security + losses
diff --git a/policyengine_us/variables/gov/states/mt/tax/income/credits/mt_elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_net_household_income.py b/policyengine_us/variables/gov/states/mt/tax/income/credits/mt_elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_net_household_income.py
index 4c996aded7b..a46ccf6f5c6 100644
--- a/policyengine_us/variables/gov/states/mt/tax/income/credits/mt_elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_net_household_income.py
+++ b/policyengine_us/variables/gov/states/mt/tax/income/credits/mt_elderly_homeowner_or_renter/mt_elderly_homeowner_or_renter_credit_net_household_income.py
@@ -8,6 +8,11 @@ class mt_elderly_homeowner_or_renter_credit_net_household_income(Variable):
     unit = USD
     definition_period = YEAR
     defined_for = "mt_elderly_homeowner_or_renter_credit_eligible"
+    reference = (
+        "https://mca.legmt.gov/bills/mca/title_0150/chapter_0300/part_0230/section_0370/0150-0300-0230-0370.html",
+        # 2024 Form 2, Schedule 2EC
+        "https://revenue.mt.gov/files/forms/Montana-Individual-Income-Tax-Return-Form-2/2024_Montana_Individual_Income_Tax_Return_Form_2.pdf#page=10",
+    )
 
     def formula(person, period, parameters):
         p = parameters(
@@ -16,10 +21,12 @@ def formula(person, period, parameters):
         # Only one claim is allowed per household
         # married taxpayer who are living apart may qualify for only one credit per year
         standard_exclusion = p.net_household_income.standard_exclusion
-        # Allocate the income to the head
+        # Gross household income counts every member of the household,
+        # including those outside the claimant's return (§ 15-30-2337(4);
+        # 2024 Schedule 2EC line 17). Allocate it to the head.
         head = person("is_tax_unit_head", period)
         gross_household_income = add(
-            person.tax_unit,
+            person.household,
             period,
             ["mt_elderly_homeowner_or_renter_credit_gross_household_income"],
         )
diff --git a/policyengine_us/variables/gov/states/mt/tax/income/deductions/cdcc/mt_child_dependent_care_expense_deduction.py b/policyengine_us/variables/gov/states/mt/tax/income/deductions/cdcc/mt_child_dependent_care_expense_deduction.py
index 2efb5ad2407..229fe2db8e6 100644
--- a/policyengine_us/variables/gov/states/mt/tax/income/deductions/cdcc/mt_child_dependent_care_expense_deduction.py
+++ b/policyengine_us/variables/gov/states/mt/tax/income/deductions/cdcc/mt_child_dependent_care_expense_deduction.py
@@ -7,9 +7,16 @@ class mt_child_dependent_care_expense_deduction(Variable):
     label = "Montana child dependent care expense deduction"
     unit = USD
     definition_period = YEAR
+    documentation = (
+        "The deduction in each filer's column: an unmarried filer's return, or "
+        "each spouse's column when spouses file separately on the same form. "
+        "A joint return uses mt_child_dependent_care_expense_deduction_joint."
+    )
     reference = (
         "https://mca.legmt.gov/bills/2019/mca/title_0150/chapter_0300/part_0210/section_0310/0150-0300-0210-0310.html",
         "https://web.archive.org/web/20230914140156/https://mtrevenue.gov/wp-content/uploads/dlm_uploads/2022/12/2441-M_2022.pdf#page=1",
+        # 2022 Form 2 instructions, Itemized Deductions Schedule, line 14
+        "https://revenue.mt.gov/files/forms/Montana-Individual-Income-Tax-Return-Form-2-Instructions/2022_Montana_Individual_Income_Tax_Return_Form_2_Instructions.pdf#page=35",
     )
     defined_for = StateCode.MT
 
@@ -19,26 +26,26 @@ def formula(person, period, parameters):
         ).gov.states.mt.tax.income.deductions.child_dependent_care_expense
         if not p.in_effect:
             return 0
-        tax_unit = person.tax_unit
-        members = tax_unit.members
-        qualifying_individual = members(
-            "mt_child_dependent_care_expense_deduction_qualifying_individual",
-            period,
-        )
-        # Form 2441-M line 1
-        qualifying_individuals = tax_unit.sum(qualifying_individual)
-        qualifying_care_expenses = tax_unit.sum(
-            members("care_expenses", period) * qualifying_individual
+        # Form 2441-M line 2
+        eligible_expenses = person.tax_unit(
+            "mt_child_dependent_care_expense_deduction_eligible_expenses", period
         )
-        childcare_expenses = tax_unit("tax_unit_childcare_expenses", period)
-        total_expenses = childcare_expenses + qualifying_care_expenses
-        # Form 2441-M line 2: the lesser of actual dependent care expenses
-        # or the cap based on the number of qualifying individuals.
-        capped_expenses = min_(total_expenses, p.cap.calc(qualifying_individuals))
-        # Line 3
-        agi = person("mt_agi_indiv", period)
-        # Line 6
-        reduction = p.phase_out.calc(agi)
-        # The deduction has to be allocated equally between spouses
+        # Line 3: add the Montana adjusted gross income in columns A and B,
+        # that is, the couple's combined income, not each spouse's own.
         head_or_spouse = person("is_tax_unit_head_or_spouse", period)
-        return head_or_spouse * (max_(0, capped_expenses - reduction) * 0.5)
+        combined_agi = person.tax_unit.sum(
+            head_or_spouse * person("mt_agi_indiv", period)
+        )
+        # Lines 4-7
+        deduction = max_(eligible_expenses - p.phase_out.calc(combined_agi), 0)
+        # Spouses filing separately on the same form each enter one-half of
+        # line 7 in their column; an unmarried filer enters all of it.
+        married = person.tax_unit("tax_unit_married", period)
+        head = person("is_tax_unit_head", period)
+        share = where(married, head_or_spouse * 0.5, head * 1.0)
+        # Married couples must file a joint return or file separately on the
+        # same form (MCA 15-30-2131(1)(c)(vi)(A)); a spouse filing on a
+        # separate form cannot take the deduction.
+        filing_status = person.tax_unit("filing_status", period)
+        separate_form = filing_status == filing_status.possible_values.SEPARATE
+        return deduction * share * ~separate_form
diff --git a/policyengine_us/variables/gov/states/mt/tax/income/deductions/cdcc/mt_child_dependent_care_expense_deduction_eligible_expenses.py b/policyengine_us/variables/gov/states/mt/tax/income/deductions/cdcc/mt_child_dependent_care_expense_deduction_eligible_expenses.py
new file mode 100644
index 00000000000..642214c3e55
--- /dev/null
+++ b/policyengine_us/variables/gov/states/mt/tax/income/deductions/cdcc/mt_child_dependent_care_expense_deduction_eligible_expenses.py
@@ -0,0 +1,34 @@
+from policyengine_us.model_api import *
+
+
+class mt_child_dependent_care_expense_deduction_eligible_expenses(Variable):
+    value_type = float
+    entity = TaxUnit
+    label = "Montana child and dependent care expenses eligible for the deduction"
+    unit = USD
+    definition_period = YEAR
+    reference = (
+        "https://mca.legmt.gov/bills/2019/mca/title_0150/chapter_0300/part_0210/section_0310/0150-0300-0210-0310.html",
+        "https://web.archive.org/web/20230914140156/https://mtrevenue.gov/wp-content/uploads/dlm_uploads/2022/12/2441-M_2022.pdf#page=1",
+    )
+    defined_for = StateCode.MT
+
+    def formula(tax_unit, period, parameters):
+        p = parameters(
+            period
+        ).gov.states.mt.tax.income.deductions.child_dependent_care_expense
+        person = tax_unit.members
+        qualifying_individual = person(
+            "mt_child_dependent_care_expense_deduction_qualifying_individual",
+            period,
+        )
+        # Form 2441-M line 1
+        qualifying_individuals = tax_unit.sum(qualifying_individual)
+        qualifying_care_expenses = tax_unit.sum(
+            person("care_expenses", period) * qualifying_individual
+        )
+        childcare_expenses = tax_unit("tax_unit_childcare_expenses", period)
+        total_expenses = childcare_expenses + qualifying_care_expenses
+        # Form 2441-M line 2: the lesser of actual dependent care expenses
+        # or the cap based on the number of qualifying individuals.
+        return min_(total_expenses, p.cap.calc(qualifying_individuals))
diff --git a/policyengine_us/variables/gov/states/mt/tax/income/deductions/cdcc/mt_child_dependent_care_expense_deduction_joint.py b/policyengine_us/variables/gov/states/mt/tax/income/deductions/cdcc/mt_child_dependent_care_expense_deduction_joint.py
new file mode 100644
index 00000000000..967ce181cb0
--- /dev/null
+++ b/policyengine_us/variables/gov/states/mt/tax/income/deductions/cdcc/mt_child_dependent_care_expense_deduction_joint.py
@@ -0,0 +1,35 @@
+from policyengine_us.model_api import *
+
+
+class mt_child_dependent_care_expense_deduction_joint(Variable):
+    value_type = float
+    entity = TaxUnit
+    label = "Montana child dependent care expense deduction on a joint return"
+    unit = USD
+    definition_period = YEAR
+    reference = (
+        "https://mca.legmt.gov/bills/2019/mca/title_0150/chapter_0300/part_0210/section_0310/0150-0300-0210-0310.html",
+        "https://web.archive.org/web/20230914140156/https://mtrevenue.gov/wp-content/uploads/dlm_uploads/2022/12/2441-M_2022.pdf#page=1",
+        # 2022 Form 2 instructions, Itemized Deductions Schedule, line 14
+        "https://revenue.mt.gov/files/forms/Montana-Individual-Income-Tax-Return-Form-2-Instructions/2022_Montana_Individual_Income_Tax_Return_Form_2_Instructions.pdf#page=35",
+    )
+    defined_for = StateCode.MT
+
+    def formula(tax_unit, period, parameters):
+        p = parameters(
+            period
+        ).gov.states.mt.tax.income.deductions.child_dependent_care_expense
+        if not p.in_effect:
+            return 0
+        # Form 2441-M line 2
+        eligible_expenses = tax_unit(
+            "mt_child_dependent_care_expense_deduction_eligible_expenses", period
+        )
+        # Line 3: the return's Montana adjusted gross income; lines 4-7.
+        agi = tax_unit("mt_agi_joint", period)
+        deduction = max_(eligible_expenses - p.phase_out.calc(agi), 0)
+        # A married person filing on a separate form cannot take the deduction
+        # (MCA 15-30-2131(1)(c)(vi)(A)).
+        filing_status = tax_unit("filing_status", period)
+        separate_form = filing_status == filing_status.possible_values.SEPARATE
+        return deduction * ~separate_form
diff --git a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_federal_income_tax_deduction_for_federal_itemization_indiv.py b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_federal_income_tax_deduction_for_federal_itemization_indiv.py
index 94951d1ad9c..967ce5bdfb5 100644
--- a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_federal_income_tax_deduction_for_federal_itemization_indiv.py
+++ b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_federal_income_tax_deduction_for_federal_itemization_indiv.py
@@ -7,28 +7,24 @@ class mt_federal_income_tax_deduction_for_federal_itemization_indiv(Variable):
     definition_period = YEAR
     label = "Montana federal income tax deduction when married couples are filing separately"
     reference = (
-        "https://law.justia.com/codes/montana/2021/title-15/chapter-30/part-21/section-15-30-2131/"
+        "https://law.justia.com/codes/montana/2021/title-15/chapter-30/part-21/section-15-30-2131/",
         # MT Code § 15-30-2131 (2021) (1)(b)
+        # 2022 Form 2 instructions, Itemized Deductions Schedule, line 4
+        "https://revenue.mt.gov/files/forms/Montana-Individual-Income-Tax-Return-Form-2-Instructions/2022_Montana_Individual_Income_Tax_Return_Form_2_Instructions.pdf#page=33",
     )
     unit = USD
     defined_for = "mt_married_filing_separately_on_same_return_eligible"
 
     def formula(person, period, parameters):
-        unit_deds = person.tax_unit(
-            "mt_federal_income_tax_deduction_for_federal_itemization", period
-        )
-        person_agi = person("mt_agi_indiv", period)
-        total_agi = person.tax_unit.sum(person_agi)
-
-        prorate = np.zeros_like(total_agi)
-        mask = total_agi > 0
-        prorate[mask] = person_agi[mask] / total_agi[mask]
+        p = parameters(
+            period
+        ).gov.states.mt.tax.income.deductions.itemized.federal_income_tax
+        # The withholding estimate is computed on each person's own income, so
+        # each spouse deducts their own amount, up to their own cap.
+        head_or_spouse = person("is_tax_unit_head_or_spouse", period)
+        withheld_income_tax = head_or_spouse * person("mt_withheld_income_tax", period)
         filing_status = person.tax_unit(
             "state_filing_status_if_married_filing_separately_on_same_return",
             period,
         )
-        p = parameters(
-            period
-        ).gov.states.mt.tax.income.deductions.itemized.federal_income_tax
-        cap = p.cap[filing_status]
-        return min_(unit_deds * prorate, cap)
+        return min_(withheld_income_tax, p.cap[filing_status])
diff --git a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_indiv.py b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_indiv.py
index 5d1cb0c5153..0d5e7d0e577 100644
--- a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_indiv.py
+++ b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_indiv.py
@@ -29,6 +29,7 @@ def formula(person, period, parameters):
             period,
             [
                 "mt_misc_deductions",
+                "mt_child_dependent_care_expense_deduction",
                 "mt_medical_expense_deduction_indiv",
                 "mt_salt_deduction",
                 "mt_federal_income_tax_deduction_for_federal_itemization_indiv",
diff --git a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_joint.py b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_joint.py
index 3469bb0b4b3..ed3371fd0ab 100644
--- a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_joint.py
+++ b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_itemization/mt_itemized_deductions_for_federal_itemization_joint.py
@@ -24,6 +24,7 @@ def formula(person, period, parameters):
             period,
             [
                 "mt_misc_deductions",
+                "mt_child_dependent_care_expense_deduction_joint",
                 "mt_medical_expense_deduction_joint",
                 "mt_salt_deduction",
                 "mt_federal_income_tax_deduction_for_federal_itemization",
diff --git a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_tax/mt_federal_income_tax_deduction_indiv.py b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_tax/mt_federal_income_tax_deduction_indiv.py
index 792174d9142..f84c008f403 100644
--- a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_tax/mt_federal_income_tax_deduction_indiv.py
+++ b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_tax/mt_federal_income_tax_deduction_indiv.py
@@ -7,26 +7,31 @@ class mt_federal_income_tax_deduction_indiv(Variable):
     definition_period = YEAR
     label = "Montana federal income tax deduction when married couples are filing separately"
     reference = (
-        "https://law.justia.com/codes/montana/2021/title-15/chapter-30/part-21/section-15-30-2131/"
+        "https://law.justia.com/codes/montana/2021/title-15/chapter-30/part-21/section-15-30-2131/",
         # MT Code § 15-30-2131 (2021) (1)(b)
+        # 2022 Form 2 instructions, Itemized Deductions Schedule, line 4
+        "https://revenue.mt.gov/files/forms/Montana-Individual-Income-Tax-Return-Form-2-Instructions/2022_Montana_Individual_Income_Tax_Return_Form_2_Instructions.pdf#page=33",
     )
     unit = USD
     defined_for = "mt_married_filing_separately_on_same_return_eligible"
 
     def formula(person, period, parameters):
-        unit_deds = person.tax_unit("mt_federal_income_tax_deduction_unit", period)
-        person_agi = person("mt_agi_indiv", period)
-        total_agi = person.tax_unit.sum(person_agi)
-
-        prorate = np.zeros_like(total_agi)
-        mask = total_agi > 0
-        prorate[mask] = person_agi[mask] / total_agi[mask]
+        p = parameters(
+            period
+        ).gov.states.mt.tax.income.deductions.itemized.federal_income_tax
+        if not p.applies:
+            return 0
+        # Federal income tax is reported by the spouse who earned the income
+        # it was paid on, and each spouse is held to their own cap; the
+        # deduction cannot be arbitrarily allocated. Without each spouse's
+        # federal withholding, attribute the couple's federal income tax by
+        # each spouse's share of federal adjusted gross income, then cap it.
+        federal_income_tax = max_(
+            person.tax_unit("income_tax_before_refundable_credits", period), 0
+        )
+        share = person("mt_federal_income_tax_deduction_share", period)
         filing_status = person.tax_unit(
             "state_filing_status_if_married_filing_separately_on_same_return",
             period,
         )
-        p = parameters(
-            period
-        ).gov.states.mt.tax.income.deductions.itemized.federal_income_tax
-        cap = p.cap[filing_status]
-        return min_(unit_deds * prorate, cap)
+        return min_(federal_income_tax * share, p.cap[filing_status])
diff --git a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_tax/mt_federal_income_tax_deduction_share.py b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_tax/mt_federal_income_tax_deduction_share.py
new file mode 100644
index 00000000000..2c6dfd8556b
--- /dev/null
+++ b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/federal_tax/mt_federal_income_tax_deduction_share.py
@@ -0,0 +1,33 @@
+from policyengine_us.model_api import *
+
+
+class mt_federal_income_tax_deduction_share(Variable):
+    value_type = float
+    entity = Person
+    definition_period = YEAR
+    label = "Share of the couple's federal income tax attributed to each spouse for the Montana deduction"
+    reference = (
+        # 2022 Form 2 instructions, Itemized Deductions Schedule, line 4a:
+        # "If you are married filing separately with your spouse, the federal
+        # income tax withheld should be reported by the spouse who earned the
+        # income."
+        "https://revenue.mt.gov/files/forms/Montana-Individual-Income-Tax-Return-Form-2-Instructions/2022_Montana_Individual_Income_Tax_Return_Form_2_Instructions.pdf#page=33",
+    )
+    unit = "/1"
+    defined_for = "mt_married_filing_separately_on_same_return_eligible"
+
+    def formula(person, period, parameters):
+        # Each filer's share of the federal income the tax was paid on,
+        # counting only positive federal adjusted gross income. A dependent's
+        # income is on the dependent's own return. With no positive income,
+        # the tax is attributed to the head.
+        head_or_spouse = person("is_tax_unit_head_or_spouse", period)
+        income = head_or_spouse * max_(
+            person("adjusted_gross_income_person", period), 0
+        )
+        total_income = person.tax_unit.sum(income)
+        share = np.zeros_like(total_income)
+        mask = total_income > 0
+        share[mask] = income[mask] / total_income[mask]
+        head = person("is_tax_unit_head", period)
+        return where(total_income > 0, share, head)
diff --git a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/general/mt_itemized_deductions_indiv.py b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/general/mt_itemized_deductions_indiv.py
index 7b31319e265..ff75bc2277f 100644
--- a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/general/mt_itemized_deductions_indiv.py
+++ b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/general/mt_itemized_deductions_indiv.py
@@ -31,6 +31,7 @@ def formula(person, period, parameters):
             period,
             [
                 "mt_misc_deductions",
+                "mt_child_dependent_care_expense_deduction",
                 "mt_medical_expense_deduction_indiv",
                 "mt_salt_deduction",
                 "mt_federal_income_tax_deduction_indiv",
diff --git a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/general/mt_itemized_deductions_joint.py b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/general/mt_itemized_deductions_joint.py
index a3328b0290d..1a13c3a97de 100644
--- a/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/general/mt_itemized_deductions_joint.py
+++ b/policyengine_us/variables/gov/states/mt/tax/income/deductions/itemized/general/mt_itemized_deductions_joint.py
@@ -24,6 +24,7 @@ def formula(person, period, parameters):
             period,
             [
                 "mt_misc_deductions",
+                "mt_child_dependent_care_expense_deduction_joint",
                 "mt_medical_expense_deduction_joint",
                 "mt_salt_deduction",
                 "mt_federal_income_tax_deduction_unit",