diff --git a/spec/integ/sliding-sync.spec.ts b/spec/integ/sliding-sync.spec.ts
index 9cf6ff2e9cd..8c4a7ad1254 100644
--- a/spec/integ/sliding-sync.spec.ts
+++ b/spec/integ/sliding-sync.spec.ts
@@ -562,6 +562,225 @@ describe("SlidingSync", () => {
         });
     });
 
+    describe("transaction IDs", () => {
+        beforeAll(setupClient);
+        afterAll(teardownClient);
+        const roomId = "!foo:bar";
+
+        let slidingSync: SlidingSync;
+
+        // really this applies to them all but it's easier to just test one
+        it("should resolve modifyRoomSubscriptions after SlidingSync.start() is called", async () => {
+            const roomSubInfo = {
+                timeline_limit: 1,
+                required_state: [
+                    ["m.room.name", ""],
+                ],
+            };
+            // add the subscription
+            slidingSync = new SlidingSync(proxyBaseUrl, [], roomSubInfo, client, 1);
+            // modification before SlidingSync.start()
+            const subscribePromise = slidingSync.modifyRoomSubscriptions(new Set([roomId]));
+            let txnId;
+            httpBackend.when("POST", syncUrl).check(function(req) {
+                const body = req.data;
+                logger.debug("got ", body);
+                expect(body.room_subscriptions).toBeTruthy();
+                expect(body.room_subscriptions[roomId]).toEqual(roomSubInfo);
+                expect(body.txn_id).toBeTruthy();
+                txnId = body.txn_id;
+            }).respond(200, function() {
+                return {
+                    pos: "aaa",
+                    txn_id: txnId,
+                    lists: [],
+                    extensions: {},
+                    rooms: {
+                        [roomId]: {
+                            name: "foo bar",
+                            required_state: [],
+                            timeline: [],
+                        },
+                    },
+                };
+            });
+            slidingSync.start();
+            await httpBackend.flushAllExpected();
+            await subscribePromise;
+        });
+        it("should resolve setList during a connection", async () => {
+            const newList = {
+                ranges: [[0, 20]],
+            };
+            const promise = slidingSync.setList(0, newList);
+            let txnId;
+            httpBackend.when("POST", syncUrl).check(function(req) {
+                const body = req.data;
+                logger.debug("got ", body);
+                expect(body.room_subscriptions).toBeFalsy();
+                expect(body.lists[0]).toEqual(newList);
+                expect(body.txn_id).toBeTruthy();
+                txnId = body.txn_id;
+            }).respond(200, function() {
+                return {
+                    pos: "bbb",
+                    txn_id: txnId,
+                    lists: [{ count: 5 }],
+                    extensions: {},
+                };
+            });
+            await httpBackend.flushAllExpected();
+            await promise;
+            expect(txnId).toBeDefined();
+        });
+        it("should resolve setListRanges during a connection", async () => {
+            const promise = slidingSync.setListRanges(0, [[20, 40]]);
+            let txnId;
+            httpBackend.when("POST", syncUrl).check(function(req) {
+                const body = req.data;
+                logger.debug("got ", body);
+                expect(body.room_subscriptions).toBeFalsy();
+                expect(body.lists[0]).toEqual({
+                    ranges: [[20, 40]],
+                });
+                expect(body.txn_id).toBeTruthy();
+                txnId = body.txn_id;
+            }).respond(200, function() {
+                return {
+                    pos: "ccc",
+                    txn_id: txnId,
+                    lists: [{ count: 5 }],
+                    extensions: {},
+                };
+            });
+            await httpBackend.flushAllExpected();
+            await promise;
+            expect(txnId).toBeDefined();
+        });
+        it("should resolve modifyRoomSubscriptionInfo during a connection", async () => {
+            const promise = slidingSync.modifyRoomSubscriptionInfo({
+                timeline_limit: 99,
+            });
+            let txnId;
+            httpBackend.when("POST", syncUrl).check(function(req) {
+                const body = req.data;
+                logger.debug("got ", body);
+                expect(body.room_subscriptions).toBeTruthy();
+                expect(body.room_subscriptions[roomId]).toEqual({
+                    timeline_limit: 99,
+                });
+                expect(body.txn_id).toBeTruthy();
+                txnId = body.txn_id;
+            }).respond(200, function() {
+                return {
+                    pos: "ddd",
+                    txn_id: txnId,
+                    extensions: {},
+                };
+            });
+            await httpBackend.flushAllExpected();
+            await promise;
+            expect(txnId).toBeDefined();
+        });
+        it("should reject earlier pending promises if a later transaction is acknowledged", async () => {
+            // i.e if we have [A,B,C] and see txn_id=C then A,B should be rejected.
+            const gotTxnIds = [];
+            const pushTxn = function(req) {
+                gotTxnIds.push(req.data.txn_id);
+            };
+            const failPromise = slidingSync.setListRanges(0, [[20, 40]]);
+            httpBackend.when("POST", syncUrl).check(pushTxn).respond(200, { pos: "e" }); // missing txn_id
+            await httpBackend.flushAllExpected();
+            const failPromise2 = slidingSync.setListRanges(0, [[60, 70]]);
+            httpBackend.when("POST", syncUrl).check(pushTxn).respond(200, { pos: "f" }); // missing txn_id
+            await httpBackend.flushAllExpected();
+
+            // attach rejection handlers now else if we do it later Jest treats that as an unhandled rejection
+            // which is a fail.
+            expect(failPromise).rejects.toEqual(gotTxnIds[0]);
+            expect(failPromise2).rejects.toEqual(gotTxnIds[1]);
+
+            const okPromise = slidingSync.setListRanges(0, [[0, 20]]);
+            let txnId;
+            httpBackend.when("POST", syncUrl).check((req) => {
+                txnId = req.data.txn_id;
+            }).respond(200, () => {
+                // include the txn_id, earlier requests should now be reject()ed.
+                return {
+                    pos: "g",
+                    txn_id: txnId,
+                };
+            });
+            await httpBackend.flushAllExpected();
+            await okPromise;
+
+            expect(txnId).toBeDefined();
+        });
+        it("should not reject later pending promises if an earlier transaction is acknowledged", async () => {
+            // i.e if we have [A,B,C] and see txn_id=B then C should not be rejected but A should.
+            const gotTxnIds = [];
+            const pushTxn = function(req) {
+                gotTxnIds.push(req.data.txn_id);
+            };
+            const A = slidingSync.setListRanges(0, [[20, 40]]);
+            httpBackend.when("POST", syncUrl).check(pushTxn).respond(200, { pos: "A" });
+            await httpBackend.flushAllExpected();
+            const B = slidingSync.setListRanges(0, [[60, 70]]);
+            httpBackend.when("POST", syncUrl).check(pushTxn).respond(200, { pos: "B" }); // missing txn_id
+            await httpBackend.flushAllExpected();
+
+            // attach rejection handlers now else if we do it later Jest treats that as an unhandled rejection
+            // which is a fail.
+            expect(A).rejects.toEqual(gotTxnIds[0]);
+
+            const C = slidingSync.setListRanges(0, [[0, 20]]);
+            let pendingC = true;
+            C.finally(() => {
+                pendingC = false;
+            });
+            httpBackend.when("POST", syncUrl).check(pushTxn).respond(200, () => {
+                // include the txn_id for B, so C's promise is outstanding
+                return {
+                    pos: "C",
+                    txn_id: gotTxnIds[1],
+                };
+            });
+            await httpBackend.flushAllExpected();
+            // A is rejected, see above
+            expect(B).resolves.toEqual(gotTxnIds[1]); // B is resolved
+            expect(pendingC).toBe(true); // C is pending still
+        });
+        it("should do nothing for unknown txn_ids", async () => {
+            const promise = slidingSync.setListRanges(0, [[20, 40]]);
+            let pending = true;
+            promise.finally(() => {
+                pending = false;
+            });
+            let txnId;
+            httpBackend.when("POST", syncUrl).check(function(req) {
+                const body = req.data;
+                logger.debug("got ", body);
+                expect(body.room_subscriptions).toBeFalsy();
+                expect(body.lists[0]).toEqual({
+                    ranges: [[20, 40]],
+                });
+                expect(body.txn_id).toBeTruthy();
+                txnId = body.txn_id;
+            }).respond(200, function() {
+                return {
+                    pos: "ccc",
+                    txn_id: "bogus transaction id",
+                    lists: [{ count: 5 }],
+                    extensions: {},
+                };
+            });
+            await httpBackend.flushAllExpected();
+            expect(txnId).toBeDefined();
+            expect(pending).toBe(true);
+            slidingSync.stop();
+        });
+    });
+
     describe("extensions", () => {
         beforeAll(setupClient);
         afterAll(teardownClient);
diff --git a/src/sliding-sync.ts b/src/sliding-sync.ts
index 5254a077dcf..28026b3a999 100644
--- a/src/sliding-sync.ts
+++ b/src/sliding-sync.ts
@@ -19,7 +19,7 @@ import { IAbortablePromise } from "./@types/partials";
 import { MatrixClient } from "./client";
 import { IRoomEvent, IStateEvent } from "./sync-accumulator";
 import { TypedEventEmitter } from "./models//typed-event-emitter";
-import { sleep } from "./utils";
+import { sleep, IDeferred, defer } from "./utils";
 
 // /sync requests allow you to set a timeout= but the request may continue
 // beyond that and wedge forever, so we need to track how long we are willing
@@ -68,6 +68,7 @@ export interface MSC3575SlidingSyncRequest {
     unsubscribe_rooms?: string[];
     room_subscriptions?: Record<string, MSC3575RoomSubscription>;
     extensions?: object;
+    txn_id?: string;
 
     // query params
     pos?: string;
@@ -126,6 +127,7 @@ type Operation = DeleteOperation | InsertOperation | InvalidateOperation | SyncO
  */
 export interface MSC3575SlidingSyncResponse {
     pos: string;
+    txn_id?: string;
     lists: ListResponse[];
     rooms: Record<string, MSC3575RoomData>;
     extensions: object;
@@ -334,6 +336,11 @@ export class SlidingSync extends TypedEventEmitter<SlidingSyncEvent, SlidingSync
     private terminated = false;
     // flag set when resend() is called because we cannot rely on detecting AbortError in JS SDK :(
     private needsResend = false;
+    // the txn_id to send with the next request.
+    private txnId?: string = null;
+    // a list (in chronological order of when they were sent) of objects containing the txn ID and
+    // a defer to resolve/reject depending on whether they were successfully sent or not.
+    private txnIdDefers: (IDeferred<string> & { txnId: string})[] = [];
     // map of extension name to req/resp handler
     private extensions: Record<string, Extension> = {};
 
@@ -403,10 +410,13 @@ export class SlidingSync extends TypedEventEmitter<SlidingSyncEvent, SlidingSync
      * whereas setList always will.
      * @param index The list index to modify
      * @param ranges The new ranges to apply.
+     * @return A promise which resolves to the transaction ID when it has been received down sync
+     * (or rejects with the transaction ID if the action was not applied e.g the request was cancelled
+     * immediately after sending, in which case the action will be applied in the subsequent request)
      */
-    public setListRanges(index: number, ranges: number[][]): void {
+    public setListRanges(index: number, ranges: number[][]): Promise<string> {
         this.lists[index].updateListRange(ranges);
-        this.resend();
+        return this.resend();
     }
 
     /**
@@ -414,15 +424,18 @@ export class SlidingSync extends TypedEventEmitter<SlidingSyncEvent, SlidingSync
      * lists.
      * @param index The index to modify
      * @param list The new list parameters.
+     * @return A promise which resolves to the transaction ID when it has been received down sync
+     * (or rejects with the transaction ID if the action was not applied e.g the request was cancelled
+     * immediately after sending, in which case the action will be applied in the subsequent request)
      */
-    public setList(index: number, list: MSC3575List): void {
+    public setList(index: number, list: MSC3575List): Promise<string> {
         if (this.lists[index]) {
             this.lists[index].replaceList(list);
         } else {
             this.lists[index] = new SlidingList(list);
         }
         this.listModifiedCount += 1;
-        this.resend();
+        return this.resend();
     }
 
     /**
@@ -438,21 +451,27 @@ export class SlidingSync extends TypedEventEmitter<SlidingSyncEvent, SlidingSync
      * /sync request to resend new subscriptions. If the /sync stream has not started, this will
      * prepare the room subscriptions for when start() is called.
      * @param s The new desired room subscriptions.
+     * @return A promise which resolves to the transaction ID when it has been received down sync
+     * (or rejects with the transaction ID if the action was not applied e.g the request was cancelled
+     * immediately after sending, in which case the action will be applied in the subsequent request)
      */
-    public modifyRoomSubscriptions(s: Set<string>) {
+    public modifyRoomSubscriptions(s: Set<string>): Promise<string> {
         this.desiredRoomSubscriptions = s;
-        this.resend();
+        return this.resend();
     }
 
     /**
      * Modify which events to retrieve for room subscriptions. Invalidates all room subscriptions
      * such that they will be sent up afresh.
      * @param rs The new room subscription fields to fetch.
+     * @return A promise which resolves to the transaction ID when it has been received down sync
+     * (or rejects with the transaction ID if the action was not applied e.g the request was cancelled
+     * immediately after sending, in which case the action will be applied in the subsequent request)
      */
-    public modifyRoomSubscriptionInfo(rs: MSC3575RoomSubscription): void {
+    public modifyRoomSubscriptionInfo(rs: MSC3575RoomSubscription): Promise<string> {
         this.roomSubscriptionInfo = rs;
         this.confirmedRoomSubscriptions = new Set<string>();
-        this.resend();
+        return this.resend();
     }
 
     /**
@@ -615,11 +634,52 @@ export class SlidingSync extends TypedEventEmitter<SlidingSyncEvent, SlidingSync
     }
 
     /**
-     * Resend a Sliding Sync request. Used when something has changed in the request.
+     * Resend a Sliding Sync request. Used when something has changed in the request. Resolves with
+     * the transaction ID of this request on success. Rejects with the transaction ID of this request
+     * on failure.
      */
-    public resend(): void {
+    public resend(): Promise<string> {
+        if (this.needsResend && this.txnIdDefers.length > 0) {
+            // we already have a resend queued, so just return the same promise
+            return this.txnIdDefers[this.txnIdDefers.length-1].promise;
+        }
         this.needsResend = true;
+        this.txnId = this.client.makeTxnId();
+        const d = defer<string>();
+        this.txnIdDefers.push({
+            ...d,
+            txnId: this.txnId,
+        });
         this.pendingReq?.abort();
+        return d.promise;
+    }
+
+    private resolveTransactionDefers(txnId?: string) {
+        if (!txnId) {
+            return;
+        }
+        // find the matching index
+        let txnIndex = -1;
+        for (let i = 0; i < this.txnIdDefers.length; i++) {
+            if (this.txnIdDefers[i].txnId === txnId) {
+                txnIndex = i;
+                break;
+            }
+        }
+        if (txnIndex === -1) {
+            // this shouldn't happen; we shouldn't be seeing txn_ids for things we don't know about,
+            // whine about it.
+            logger.warn(`resolveTransactionDefers: seen ${txnId} but it isn't a pending txn, ignoring.`);
+            return;
+        }
+        // This list is sorted in time, so if the input txnId ACKs in the middle of this array,
+        // then everything before it that hasn't been ACKed yet never will and we should reject them.
+        for (let i = 0; i < txnIndex; i++) {
+            this.txnIdDefers[i].reject(this.txnIdDefers[i].txnId);
+        }
+        this.txnIdDefers[txnIndex].resolve(txnId);
+        // clear out settled promises, incuding the one we resolved.
+        this.txnIdDefers = this.txnIdDefers.slice(txnIndex+1);
     }
 
     /**
@@ -666,6 +726,10 @@ export class SlidingSync extends TypedEventEmitter<SlidingSyncEvent, SlidingSync
                         reqBody.room_subscriptions[roomId] = this.roomSubscriptionInfo;
                     }
                 }
+                if (this.txnId) {
+                    reqBody.txn_id = this.txnId;
+                    this.txnId = null;
+                }
                 this.pendingReq = this.client.slidingSync(reqBody, this.proxyBaseUrl);
                 resp = await this.pendingReq;
                 logger.debug(resp);
@@ -747,6 +811,8 @@ export class SlidingSync extends TypedEventEmitter<SlidingSyncEvent, SlidingSync
                     i, this.lists[i].joinedCount, Object.assign({}, this.lists[i].roomIndexToRoomId),
                 );
             });
+
+            this.resolveTransactionDefers(resp.txn_id);
         }
     }
 }