diff --git a/README.md b/README.md
index f0256adb..2a2979d8 100644
--- a/README.md
+++ b/README.md
@@ -163,6 +163,32 @@ const event3 = new CloudEvent({
 });
 ```
 
+### Extension Attributes
+
+Any property passed to the `CloudEvent` constructor that is not a core attribute becomes an
+[extension attribute](https://github.com/cloudevents/spec/blob/v1.0/spec.md#extension-context-attributes).
+In binary mode each extension is sent as its own header, named after the attribute with the
+transport's prefix: an extension `traceid` becomes the `ce-traceid` HTTP header. Extension names
+must be lowercase letters and digits only.
+
+Extension values should be one of the CloudEvents types: string, boolean, integer, binary, URI or
+timestamp. The spec has no object type, so encode structured data as a string yourself:
+
+```js
+const ce = new CloudEvent({
+  source: "/orders",
+  type: "order.placed",
+  traceinfo: JSON.stringify({ span: "abc", sampled: true }),
+});
+```
+
+`Date` values are sent as RFC 3339 timestamps and binary values (`Buffer`, typed arrays) as base64, which
+are the string forms the spec requires.
+
+As a convenience the SDK also accepts a plain object or array as an extension value and serializes it as JSON
+when the event is sent in binary mode. This is not spec-conformant: a receiver gets a plain string
+and has to know to parse it, and other SDKs may reject the event. Prefer the explicit form above.
+
 ### A Note About Big Integers
 
 When parsing JSON data, if a JSON field value is a number, and that number
diff --git a/src/event/interfaces.ts b/src/event/interfaces.ts
index f73c6d8f..72d3843b 100644
--- a/src/event/interfaces.ts
+++ b/src/event/interfaces.ts
@@ -136,7 +136,19 @@ export interface CloudEventV1OptionalAttributes<T> {
   data_base64?: string;
 
   /**
-   * [OPTIONAL] CloudEvents extension attributes.
+   * [OPTIONAL] CloudEvents extension attributes. Any property that is not a
+   * core attribute is treated as an extension. Values should be one of the
+   * CloudEvents types: string, boolean, integer, binary, URI or timestamp.
+   * In binary mode a Date is sent as RFC 3339 and binary values as base64.
+   * 
+   * Object values are accepted as a convenience and for compatibility reasons. 
+   * They are serialized as JSON in binary mode, but this is not spec-conformant; 
+   * prefer encoding the value as a string yourself. 
+   * 
+   * @deprecated automatic serialization of object values is not spec-conformant 
+   * and may be removed in future versions of the SDK.
+   * 
+   * @see https://github.com/cloudevents/spec/blob/v1.0/spec.md#extension-context-attributes
    */
   [key: string]: unknown;
 }
diff --git a/src/message/http/headers.ts b/src/message/http/headers.ts
index b0cc519b..15fb15e4 100644
--- a/src/message/http/headers.ts
+++ b/src/message/http/headers.ts
@@ -8,6 +8,7 @@ import { CloudEventV1 } from "../..";
 import { Headers } from "../";
 import { V1 } from "../../event/cloudevent";
 import CONSTANTS from "../../constants";
+import { asBase64, isBinary, isDate, TypeArray } from "../../event/validation";
 
 export const allowedContentTypes = [CONSTANTS.DEFAULT_CONTENT_TYPE, CONSTANTS.MIME_JSON, CONSTANTS.MIME_OCTET_STREAM];
 export const requiredHeaders = [
@@ -41,7 +42,7 @@ export function headersFor<T>(event: CloudEventV1<T>): Headers {
       if (map) {
         headers[map.name] = map.parser.parse(value as string) as string;
       } else if (property !== CONSTANTS.DATA_ATTRIBUTE && property !== `${CONSTANTS.DATA_ATTRIBUTE}_base64`) {
-        headers[`${CONSTANTS.EXTENSIONS_PREFIX}${property}`] = value as string;
+        headers[`${CONSTANTS.EXTENSIONS_PREFIX}${property}`] = stringifyExtensionValue(value);
       }
     }
   });
@@ -52,6 +53,55 @@ export function headersFor<T>(event: CloudEventV1<T>): Headers {
   return headers;
 }
 
+/**
+ * True for objects created by a literal, `new Object()` or `Object.create(null)`.
+ * The check looks at the prototype, so it is not fooled by a shadowed `constructor`
+ * property and it does not depend on `Object.prototype.toString`, which a
+ * Symbol.toStringTag can spoof. Class instances (Error, Map, Date, ...) are not plain.
+ * @param {unknown} value the value to test
+ * @returns {boolean} true if the value is a plain object
+ */
+const isPlainObject = (value: unknown): value is Record<string, unknown> =>
+  value !== null &&
+  typeof value === "object" &&
+  (Object.getPrototypeOf(value) === Object.prototype ||
+    (Object.getPrototypeOf(value) === null && Object.prototype.toString.call(value) === "[object Object]"));
+
+/**
+ * Converts an extension attribute value to its binary-mode header value.
+ *
+ * The CloudEvents type system has no object type: extension values are strings,
+ * booleans, integers, binary, URIs or timestamps. Values that are not already
+ * strings are converted as follows; the branches are disjoint, so their order
+ * does not matter. The last column is what the HTTP client's String() coercion
+ * used to put on the wire.
+ *
+ * | value                    | header value            | before                          |
+ * |--------------------------|-------------------------|---------------------------------|
+ * | Date                     | RFC 3339 (toISOString)  | local-time string               |
+ * | Buffer, typed array      | base64                  | UTF-8 text, or "104,105"        |
+ * | plain object, array      | JSON (convenience)      | "[object Object]", or "1,2"     |
+ * | anything else            | unchanged               | unchanged                       |
+ *
+ * JSON for plain objects and arrays is an SDK convenience, not spec behavior;
+ * prefer encoding such values as strings yourself. Anything else (Error, Map,
+ * null, ...) is not a CloudEvents type and is left for the HTTP client, as before.
+ * @param {unknown} value the extension attribute value
+ * @returns {string} the header value
+ */
+function stringifyExtensionValue(value: unknown): string {
+  if (isDate(value)) {
+    return value.toISOString();
+  }
+  if (isBinary(value)) {
+    return asBase64(value as TypeArray);
+  }
+  if (isPlainObject(value) || Array.isArray(value)) {
+    return JSON.stringify(value);
+  }
+  return value as string;
+}
+
 /**
  * Sanitizes incoming headers by lowercasing them and potentially removing
  * encoding from the content-type header.
diff --git a/test/integration/message_test.ts b/test/integration/message_test.ts
index 9ddb292d..8ac2200f 100644
--- a/test/integration/message_test.ts
+++ b/test/integration/message_test.ts
@@ -65,6 +65,54 @@ describe("HTTP transport", () => {
     expect(message.headers["ce-extboolean"]).to.equal(false);
   });
 
+  it("Serializes object-valued extensions as JSON in binary mode", () => {
+    const extobject = { some: "object", nested: { answer: 42 } };
+    const evt = new CloudEvent({ source: "test", type: "test", extobject });
+    const message = HTTP.binary(evt);
+    // HTTP headers can only carry strings; a raw object reaches the wire as "[object Object]"
+    expect(message.headers["ce-extobject"]).to.equal(JSON.stringify(extobject));
+  });
+
+  it("Round-trips an object-valued extension through binary mode as a JSON string", () => {
+    const extobject = { some: "object", nested: { answer: 42 } };
+    const evt = new CloudEvent({ source: "test", type: "test", extobject });
+    const received = HTTP.toEvent(HTTP.binary(evt)) as CloudEvent;
+    // the receiver sees the serialized form and is responsible for parsing it
+    expect(received.extobject).to.equal(JSON.stringify(extobject));
+    expect(JSON.parse(received.extobject as string)).to.deep.equal(extobject);
+  });
+
+  it("Serializes Date-valued extensions as RFC 3339 timestamps in binary mode", () => {
+    const extdate = new Date("2018-04-05T03:56:24Z");
+    const message = HTTP.binary(new CloudEvent({ source: "test", type: "test", extdate }));
+    expect(message.headers["ce-extdate"]).to.equal("2018-04-05T03:56:24.000Z");
+  });
+
+  it("Serializes binary extensions as base64 in binary mode", () => {
+    const extbytes = new Uint8Array([104, 105]);
+    const extbuffer = Buffer.from("hi");
+    const message = HTTP.binary(new CloudEvent({ source: "test", type: "test", extbytes, extbuffer }));
+    expect(message.headers["ce-extbytes"]).to.equal("aGk=");
+    expect(message.headers["ce-extbuffer"]).to.equal("aGk=");
+  });
+
+  it("Serializes plain objects without an Object constructor as JSON in binary mode", () => {
+    // a shadowed `constructor` property or a null prototype must not defeat plain-object detection
+    const extshadowed = { constructor: "shadowed", some: "object" };
+    const extnullproto = Object.assign(Object.create(null), { some: "object" });
+    const message = HTTP.binary(new CloudEvent({ source: "test", type: "test", extshadowed, extnullproto }));
+    expect(message.headers["ce-extshadowed"]).to.equal(JSON.stringify(extshadowed));
+    expect(message.headers["ce-extnullproto"]).to.equal(JSON.stringify(extnullproto));
+  });
+
+  it("Leaves non-CloudEvents values in binary-mode extensions untouched", () => {
+    const exterror = new Error("useful message");
+    const message = HTTP.binary(new CloudEvent({ source: "test", type: "test", exterror }));
+    // JSON.stringify would turn the Error into "{}"; the HTTP client's String() keeps the message
+    expect(message.headers["ce-exterror"]).to.equal(exterror);
+    expect(String(message.headers["ce-exterror"])).to.equal("Error: useful message");
+  });
+
   it("Includes extensions in structured when type is 'boolean' with a false value", () => {
     const evt = new CloudEvent({ source: "test", type: "test", extboolean: false });
     expect(evt.hasOwnProperty("extboolean")).to.equal(true);