diff --git a/src/common_models/add_param_container.jl b/src/common_models/add_param_container.jl
index d9132799..088b4d89 100644
--- a/src/common_models/add_param_container.jl
+++ b/src/common_models/add_param_container.jl
@@ -101,8 +101,8 @@ function add_variable_value_parameter_container!(
 end
 
 """
-Allocate an event parameter container (`EventParametersAttributes`). `V` is the component type
-whose instances the event affects.
+Allocate an event parameter container (`EventParametersAttributes`). `U` is the component type
+the event affects; `V` is the supplemental-attribute type describing the event itself.
 """
 function add_event_parameter_container!(
     container::OptimizationContainer,
@@ -115,7 +115,7 @@ function add_event_parameter_container!(
 ) where {
     T <: EventParameter,
     U <: IS.InfrastructureSystemsComponent,
-    V <: IS.InfrastructureSystemsComponent,
+    V <: IS.SupplementalAttribute,
 }
     param_key = ParameterKey(T, U, meta)
     attributes = EventParametersAttributes(V, T)
diff --git a/src/common_models/add_param_container_shims.jl b/src/common_models/add_param_container_shims.jl
index bb3aae22..573b1fc8 100644
--- a/src/common_models/add_param_container_shims.jl
+++ b/src/common_models/add_param_container_shims.jl
@@ -86,6 +86,6 @@ add_param_container!(
 ) where {
     T <: EventParameter,
     U <: IS.InfrastructureSystemsComponent,
-    V <: IS.InfrastructureSystemsComponent,
+    V <: IS.SupplementalAttribute,
 } = add_event_parameter_container!(
     container, T, U, V, axs...; sparse = sparse, meta = meta)
diff --git a/src/core/parameter_container.jl b/src/core/parameter_container.jl
index 07de9837..9b23ce3a 100644
--- a/src/core/parameter_container.jl
+++ b/src/core/parameter_container.jl
@@ -81,25 +81,30 @@ get_variable_types(attr::CostFunctionAttributes) = attr.variable_types
 get_uses_compact_power(attr::CostFunctionAttributes) = attr.uses_compact_power
 
 struct EventParametersAttributes{
-    T <: IS.InfrastructureSystemsComponent,
+    T <: IS.SupplementalAttribute,
     U <: ParameterType,
 } <: ParameterAttributes
-    affected_devices::Vector{T}
 end
 
 function EventParametersAttributes(
     ::Type{T},
     ::Type{U},
-) where {T <: IS.InfrastructureSystemsComponent, U <: ParameterType}
-    return EventParametersAttributes{T, U}(T[])
+) where {T <: IS.SupplementalAttribute, U <: ParameterType}
+    return EventParametersAttributes{T, U}()
 end
 
 function get_param_type(
     ::EventParametersAttributes{T, U},
-) where {T <: IS.InfrastructureSystemsComponent, U <: ParameterType}
+) where {T <: IS.SupplementalAttribute, U <: ParameterType}
     return U
 end
 
+function get_attribute_type(
+    ::EventParametersAttributes{T, U},
+) where {T <: IS.SupplementalAttribute, U <: ParameterType}
+    return T
+end
+
 #################################################################################
 # Parameter Container - holds parameter arrays and their attributes
 #################################################################################
diff --git a/test/InfrastructureOptimizationModelsTests.jl b/test/InfrastructureOptimizationModelsTests.jl
index 3f6559e8..ad28b334 100644
--- a/test/InfrastructureOptimizationModelsTests.jl
+++ b/test/InfrastructureOptimizationModelsTests.jl
@@ -111,7 +111,7 @@ function run_tests()
                     # optimization_problem_outputs_export.jl: low-complexity
                     include(joinpath(TEST_DIR, "test_optimization_outputs.jl"))
                     include(joinpath(TEST_DIR, "test_optimizer_stats.jl"))
-                    # parameter_container.jl: low-complexity
+                    include(joinpath(TEST_DIR, "test_event_parameter_container.jl"))
                     # TODO outputs_by_time.jl
                     # TODO service_model.jl
                     include(joinpath(TEST_DIR, "test_settings.jl"))
diff --git a/test/mocks/mock_components.jl b/test/mocks/mock_components.jl
index d54f9310..57d7d672 100644
--- a/test/mocks/mock_components.jl
+++ b/test/mocks/mock_components.jl
@@ -146,6 +146,11 @@ get_from_bus(b::MockBranch) = b.from_bus
 get_to_bus(b::MockBranch) = b.to_bus
 get_rate(b::MockBranch) = b.rating
 
+# Mock supplemental attribute type for use as the event type parameter of event
+# parameter containers. Real event types (e.g. PSY.FixedForcedOutage) are supplemental
+# attributes, not components.
+struct MockOutageAttribute <: IS.SupplementalAttribute end
+
 # Mock component type for use as type parameter in container keys
 # This replaces PSY.ThermalStandard etc. in tests that don't need real PSY types
 # Subtypes IS.InfrastructureSystemsComponent so it works with VariableKey, ConstraintKey, etc.
diff --git a/test/test_event_parameter_container.jl b/test/test_event_parameter_container.jl
new file mode 100644
index 00000000..e8dac035
--- /dev/null
+++ b/test/test_event_parameter_container.jl
@@ -0,0 +1,70 @@
+"""
+Unit tests for the event parameter container (`EventParametersAttributes`).
+
+The type parameter describing the event is a *supplemental attribute*, not a component:
+downstream event models key parameters off types such as `PSY.FixedForcedOutage`, which
+subtype `IS.SupplementalAttribute`. Bounding that slot on `IS.InfrastructureSystemsComponent`
+makes every real call a `MethodError`, so the bound is what these tests pin down.
+"""
+
+struct EventContainerParameter <: IOM.EventParameter end
+
+function _make_event_parameter_container(time_steps)
+    mock_sys = MockSystem(100.0)
+    settings = IOM.Settings(
+        mock_sys;
+        horizon = Dates.Hour(length(time_steps)),
+        resolution = Dates.Hour(1),
+        time_series_cache_size = 0,
+    )
+    container = IOM.OptimizationContainer(mock_sys, settings, nothing, MockDeterministic)
+    IOM.set_time_steps!(container, time_steps)
+    return container
+end
+
+@testset "Event parameter container" begin
+    time_steps = 1:3
+    names = ["A", "B"]
+
+    @testset "Allocation with a supplemental-attribute event type" begin
+        container = _make_event_parameter_container(time_steps)
+        param_container = IOM.add_event_parameter_container!(
+            container,
+            EventContainerParameter,
+            MockThermalGen,
+            MockOutageAttribute,
+            names,
+            time_steps,
+        )
+        param_array = IOM.get_parameter_array(param_container)
+        @test axes(param_array)[1] == names
+        @test axes(param_array)[2] == time_steps
+        @test size(IOM.get_multiplier_array(param_container)) == (2, 3)
+
+        attributes = IOM.get_attributes(param_container)
+        @test attributes isa
+              IOM.EventParametersAttributes{MockOutageAttribute, EventContainerParameter}
+        @test IOM.get_param_type(attributes) === EventContainerParameter
+        @test IOM.get_attribute_type(attributes) === MockOutageAttribute
+
+        @test IOM.has_container_key(
+            container,
+            EventContainerParameter,
+            MockThermalGen,
+        )
+    end
+
+    @testset "The add_param_container! shim routes to the event overload" begin
+        container = _make_event_parameter_container(time_steps)
+        param_container = IOM.add_param_container!(
+            container,
+            EventContainerParameter,
+            MockThermalGen,
+            MockOutageAttribute,
+            names,
+            time_steps,
+        )
+        @test IOM.get_attributes(param_container) isa
+              IOM.EventParametersAttributes{MockOutageAttribute, EventContainerParameter}
+    end
+end
diff --git a/test/test_parameterized_range_constraints.jl b/test/test_parameterized_range_constraints.jl
index 92d9cf0a..0be0522f 100644
--- a/test/test_parameterized_range_constraints.jl
+++ b/test/test_parameterized_range_constraints.jl
@@ -183,7 +183,7 @@ end
             container,
             TestEventParameter,
             MockThermalGen,
-            MockThermalGen,
+            MockOutageAttribute,
             names,
             time_steps,
         )