diff --git a/spec/unit/matrixrtc/MatrixRTCSession.spec.ts b/spec/unit/matrixrtc/MatrixRTCSession.spec.ts
index 1d9feea13..16f55386f 100644
--- a/spec/unit/matrixrtc/MatrixRTCSession.spec.ts
+++ b/spec/unit/matrixrtc/MatrixRTCSession.spec.ts
@@ -14,7 +14,7 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import { EventTimeline, EventType, MatrixClient, MatrixError, MatrixEvent, Room } from "../../../src";
+import { encodeBase64, EventTimeline, EventType, MatrixClient, MatrixError, MatrixEvent, Room } from "../../../src";
 import { KnownMembership } from "../../../src/@types/membership";
 import {
     CallMembershipData,
@@ -1145,6 +1145,7 @@ describe("MatrixRTCSession", () => {
                 ],
             }),
             getSender: jest.fn().mockReturnValue("@bob:example.org"),
+            getTs: jest.fn().mockReturnValue(Date.now()),
         } as unknown as MatrixEvent);
 
         const bobKeys = sess.getKeysForParticipant("@bob:example.org", "bobsphone")!;
@@ -1168,6 +1169,7 @@ describe("MatrixRTCSession", () => {
                 ],
             }),
             getSender: jest.fn().mockReturnValue("@bob:example.org"),
+            getTs: jest.fn().mockReturnValue(Date.now()),
         } as unknown as MatrixEvent);
 
         const bobKeys = sess.getKeysForParticipant("@bob:example.org", "bobsphone")!;
@@ -1179,6 +1181,130 @@ describe("MatrixRTCSession", () => {
         expect(bobKeys[4]).toEqual(Buffer.from("this is the key", "utf-8"));
     });
 
+    it("collects keys by merging", () => {
+        const mockRoom = makeMockRoom([membershipTemplate]);
+        sess = MatrixRTCSession.roomSessionForRoom(client, mockRoom);
+        sess.onCallEncryption({
+            getType: jest.fn().mockReturnValue("io.element.call.encryption_keys"),
+            getContent: jest.fn().mockReturnValue({
+                device_id: "bobsphone",
+                call_id: "",
+                keys: [
+                    {
+                        index: 0,
+                        key: "dGhpcyBpcyB0aGUga2V5",
+                    },
+                ],
+            }),
+            getSender: jest.fn().mockReturnValue("@bob:example.org"),
+            getTs: jest.fn().mockReturnValue(Date.now()),
+        } as unknown as MatrixEvent);
+
+        let bobKeys = sess.getKeysForParticipant("@bob:example.org", "bobsphone")!;
+        expect(bobKeys).toHaveLength(1);
+        expect(bobKeys[0]).toEqual(Buffer.from("this is the key", "utf-8"));
+
+        sess.onCallEncryption({
+            getType: jest.fn().mockReturnValue("io.element.call.encryption_keys"),
+            getContent: jest.fn().mockReturnValue({
+                device_id: "bobsphone",
+                call_id: "",
+                keys: [
+                    {
+                        index: 4,
+                        key: "dGhpcyBpcyB0aGUga2V5",
+                    },
+                ],
+            }),
+            getSender: jest.fn().mockReturnValue("@bob:example.org"),
+            getTs: jest.fn().mockReturnValue(Date.now()),
+        } as unknown as MatrixEvent);
+
+        bobKeys = sess.getKeysForParticipant("@bob:example.org", "bobsphone")!;
+        expect(bobKeys).toHaveLength(5);
+        expect(bobKeys[4]).toEqual(Buffer.from("this is the key", "utf-8"));
+    });
+
+    it("ignores older keys at same index", () => {
+        const mockRoom = makeMockRoom([membershipTemplate]);
+        sess = MatrixRTCSession.roomSessionForRoom(client, mockRoom);
+        sess.onCallEncryption({
+            getType: jest.fn().mockReturnValue("io.element.call.encryption_keys"),
+            getContent: jest.fn().mockReturnValue({
+                device_id: "bobsphone",
+                call_id: "",
+                keys: [
+                    {
+                        index: 0,
+                        key: encodeBase64(Buffer.from("newer key", "utf-8")),
+                    },
+                ],
+            }),
+            getSender: jest.fn().mockReturnValue("@bob:example.org"),
+            getTs: jest.fn().mockReturnValue(2000),
+        } as unknown as MatrixEvent);
+
+        sess.onCallEncryption({
+            getType: jest.fn().mockReturnValue("io.element.call.encryption_keys"),
+            getContent: jest.fn().mockReturnValue({
+                device_id: "bobsphone",
+                call_id: "",
+                keys: [
+                    {
+                        index: 0,
+                        key: encodeBase64(Buffer.from("older key", "utf-8")),
+                    },
+                ],
+            }),
+            getSender: jest.fn().mockReturnValue("@bob:example.org"),
+            getTs: jest.fn().mockReturnValue(1000), // earlier timestamp than the newer key
+        } as unknown as MatrixEvent);
+
+        const bobKeys = sess.getKeysForParticipant("@bob:example.org", "bobsphone")!;
+        expect(bobKeys).toHaveLength(1);
+        expect(bobKeys[0]).toEqual(Buffer.from("newer key", "utf-8"));
+    });
+
+    it("key timestamps are treated as monotonic", () => {
+        const mockRoom = makeMockRoom([membershipTemplate]);
+        sess = MatrixRTCSession.roomSessionForRoom(client, mockRoom);
+        sess.onCallEncryption({
+            getType: jest.fn().mockReturnValue("io.element.call.encryption_keys"),
+            getContent: jest.fn().mockReturnValue({
+                device_id: "bobsphone",
+                call_id: "",
+                keys: [
+                    {
+                        index: 0,
+                        key: encodeBase64(Buffer.from("first key", "utf-8")),
+                    },
+                ],
+            }),
+            getSender: jest.fn().mockReturnValue("@bob:example.org"),
+            getTs: jest.fn().mockReturnValue(1000),
+        } as unknown as MatrixEvent);
+
+        sess.onCallEncryption({
+            getType: jest.fn().mockReturnValue("io.element.call.encryption_keys"),
+            getContent: jest.fn().mockReturnValue({
+                device_id: "bobsphone",
+                call_id: "",
+                keys: [
+                    {
+                        index: 0,
+                        key: encodeBase64(Buffer.from("second key", "utf-8")),
+                    },
+                ],
+            }),
+            getSender: jest.fn().mockReturnValue("@bob:example.org"),
+            getTs: jest.fn().mockReturnValue(1000), // same timestamp as the first key
+        } as unknown as MatrixEvent);
+
+        const bobKeys = sess.getKeysForParticipant("@bob:example.org", "bobsphone")!;
+        expect(bobKeys).toHaveLength(1);
+        expect(bobKeys[0]).toEqual(Buffer.from("second key", "utf-8"));
+    });
+
     it("ignores keys event for the local participant", () => {
         const mockRoom = makeMockRoom([membershipTemplate]);
         sess = MatrixRTCSession.roomSessionForRoom(client, mockRoom);
@@ -1195,6 +1321,7 @@ describe("MatrixRTCSession", () => {
                 ],
             }),
             getSender: jest.fn().mockReturnValue(client.getUserId()),
+            getTs: jest.fn().mockReturnValue(Date.now()),
         } as unknown as MatrixEvent);
 
         const myKeys = sess.getKeysForParticipant(client.getUserId()!, client.getDeviceId()!)!;
diff --git a/src/matrixrtc/MatrixRTCSession.ts b/src/matrixrtc/MatrixRTCSession.ts
index ed3d7a44f..c5b73a3fd 100644
--- a/src/matrixrtc/MatrixRTCSession.ts
+++ b/src/matrixrtc/MatrixRTCSession.ts
@@ -56,9 +56,9 @@ const USE_KEY_DELAY = 5000;
 const getParticipantId = (userId: string, deviceId: string): string => `${userId}:${deviceId}`;
 const getParticipantIdFromMembership = (m: CallMembership): string => getParticipantId(m.sender!, m.deviceId);
 
-function keysEqual(a: Uint8Array, b: Uint8Array): boolean {
+function keysEqual(a: Uint8Array | undefined, b: Uint8Array | undefined): boolean {
     if (a === b) return true;
-    return a && b && a.length === b.length && a.every((x, i) => x === b[i]);
+    return !!a && !!b && a.length === b.length && a.every((x, i) => x === b[i]);
 }
 
 export enum MatrixRTCSessionEvent {
@@ -134,8 +134,8 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
 
     private manageMediaKeys = false;
     private useLegacyMemberEvents = true;
-    // userId:deviceId => array of keys
-    private encryptionKeys = new Map<string, Array<Uint8Array>>();
+    // userId:deviceId => array of (key, timestamp)
+    private encryptionKeys = new Map<string, Array<{ key: Uint8Array; timestamp: number }>>();
     private lastEncryptionKeyUpdateRequest?: number;
 
     // We use this to store the last membership fingerprints we saw, so we can proactively re-send encryption keys
@@ -378,8 +378,15 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
         }
     }
 
+    /**
+     * Get the known encryption keys for a given participant device.
+     *
+     * @param userId the user ID of the participant
+     * @param deviceId the device ID of the participant
+     * @returns The encryption keys for the given participant, or undefined if they are not known.
+     */
     public getKeysForParticipant(userId: string, deviceId: string): Array<Uint8Array> | undefined {
-        return this.encryptionKeys.get(getParticipantId(userId, deviceId));
+        return this.encryptionKeys.get(getParticipantId(userId, deviceId))?.map((entry) => entry.key);
     }
 
     /**
@@ -387,7 +394,10 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
      * cipher) given participant's media. This also includes our own key
      */
     public getEncryptionKeys(): IterableIterator<[string, Array<Uint8Array>]> {
-        return this.encryptionKeys.entries();
+        // the returned array doesn't contain the timestamps
+        return Array.from(this.encryptionKeys.entries())
+            .map(([participantId, keys]): [string, Uint8Array[]] => [participantId, keys.map((k) => k.key)])
+            .values();
     }
 
     private getNewEncryptionKeyIndex(): number {
@@ -402,12 +412,14 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
 
     /**
      * Sets an encryption key at a specified index for a participant.
-     * The encryption keys for the local participanmt are also stored here under the
+     * The encryption keys for the local participant are also stored here under the
      * user and device ID of the local participant.
+     * If the key is older than the existing key at the index, it will be ignored.
      * @param userId - The user ID of the participant
      * @param deviceId - Device ID of the participant
      * @param encryptionKeyIndex - The index of the key to set
      * @param encryptionKeyString - The string representation of the key to set in base64
+     * @param timestamp - The timestamp of the key. We assume that these are monotonic for each participant device.
      * @param delayBeforeUse - If true, delay before emitting a key changed event. Useful when setting
      *                         encryption keys for the local participant to allow time for the key to
      *                         be distributed.
@@ -417,17 +429,38 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
         deviceId: string,
         encryptionKeyIndex: number,
         encryptionKeyString: string,
+        timestamp: number,
         delayBeforeUse = false,
     ): void {
         const keyBin = decodeBase64(encryptionKeyString);
 
         const participantId = getParticipantId(userId, deviceId);
-        const encryptionKeys = this.encryptionKeys.get(participantId) ?? [];
+        if (!this.encryptionKeys.has(participantId)) {
+            this.encryptionKeys.set(participantId, []);
+        }
+        const participantKeys = this.encryptionKeys.get(participantId)!;
 
-        if (keysEqual(encryptionKeys[encryptionKeyIndex], keyBin)) return;
+        const existingKeyAtIndex = participantKeys[encryptionKeyIndex];
+
+        if (existingKeyAtIndex) {
+            if (existingKeyAtIndex.timestamp > timestamp) {
+                logger.info(
+                    `Ignoring new key at index ${encryptionKeyIndex} for ${participantId} as it is older than existing known key`,
+                );
+                return;
+            }
+
+            if (keysEqual(existingKeyAtIndex.key, keyBin)) {
+                existingKeyAtIndex.timestamp = timestamp;
+                return;
+            }
+        }
+
+        participantKeys[encryptionKeyIndex] = {
+            key: keyBin,
+            timestamp,
+        };
 
-        encryptionKeys[encryptionKeyIndex] = keyBin;
-        this.encryptionKeys.set(participantId, encryptionKeys);
         if (delayBeforeUse) {
             const useKeyTimeout = setTimeout(() => {
                 this.setNewKeyTimeouts.delete(useKeyTimeout);
@@ -455,7 +488,7 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
         const encryptionKey = secureRandomBase64Url(16);
         const encryptionKeyIndex = this.getNewEncryptionKeyIndex();
         logger.info("Generated new key at index " + encryptionKeyIndex);
-        this.setEncryptionKey(userId, deviceId, encryptionKeyIndex, encryptionKey, delayBeforeUse);
+        this.setEncryptionKey(userId, deviceId, encryptionKeyIndex, encryptionKey, Date.now(), delayBeforeUse);
     }
 
     /**
@@ -574,6 +607,13 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
         }
     }
 
+    /**
+     * Process `m.call.encryption_keys` events to track the encryption keys for call participants.
+     * This should be called each time the relevant event is received from a room timeline.
+     * If the event is malformed then it will be logged and ignored.
+     *
+     * @param event the event to process
+     */
     public onCallEncryption = (event: MatrixEvent): void => {
         const userId = event.getSender();
         const content = event.getContent<EncryptionKeysEventContent>();
@@ -635,7 +675,7 @@ export class MatrixRTCSession extends TypedEventEmitter<MatrixRTCSessionEvent, M
                     `Embedded-E2EE-LOG onCallEncryption userId=${userId}:${deviceId} encryptionKeyIndex=${encryptionKeyIndex}`,
                     this.encryptionKeys,
                 );
-                this.setEncryptionKey(userId, deviceId, encryptionKeyIndex, encryptionKey);
+                this.setEncryptionKey(userId, deviceId, encryptionKeyIndex, encryptionKey, event.getTs());
             }
         }
     };