diff --git a/src/store/indexeddb.js b/src/store/indexeddb.js
index 9b671e9db..1182aa489 100644
--- a/src/store/indexeddb.js
+++ b/src/store/indexeddb.js
@@ -1,5 +1,5 @@
 /*
-Copyright 2017 OpenMarket Ltd
+Copyright 2017 Vector Creations Ltd
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
diff --git a/src/sync-accumulator.js b/src/sync-accumulator.js
new file mode 100644
index 000000000..bcaf84536
--- /dev/null
+++ b/src/sync-accumulator.js
@@ -0,0 +1,105 @@
+/*
+Copyright 2017 Vector Creations Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+"use strict";
+
+
+/**
+ * Internal class.
+ *
+ * The purpose of this class is to accumulate /sync responses such that a
+ * complete "initial" JSON response can be returned which accurately represents
+ * the sum total of the /sync responses accumulated to date. It only handles
+ * room data: that is, everything under the "rooms" top-level key.
+ *
+ * This class is used when persisting room data so a complete /sync response can
+ * be loaded from disk and incremental syncs can be performed on the server,
+ * rather than asking the server to do an initial sync on startup.
+ */
+class SyncAccumulator {
+
+    constructor() {
+        this.rooms = {
+            // $room_id : {
+            //   category: invite|join|leave,
+            //   data: { ... sync json data ... }
+            // }
+        };
+    }
+
+    /**
+     * Accumulate incremental /sync data.
+     * @param {Object} syncResponse the complete /sync JSON
+     */
+    accumulate(syncResponse) {
+        if (!syncResponse.rooms) {
+            return;
+        }
+        if (syncResponse.rooms.invite) {
+            Object.keys(syncResponse.rooms.invite).forEach((roomId) => {
+                this._accumulateRoom(
+                    roomId, "invite", syncResponse.rooms.invite[roomId],
+                );
+            });
+        }
+        if (syncResponse.rooms.join) {
+            Object.keys(syncResponse.rooms.join).forEach((roomId) => {
+                this._accumulateRoom(
+                    roomId, "join", syncResponse.rooms.join[roomId],
+                );
+            });
+        }
+        if (syncResponse.rooms.leave) {
+            Object.keys(syncResponse.rooms.leave).forEach((roomId) => {
+                this._accumulateRoom(
+                    roomId, "leave", syncResponse.rooms.leave[roomId],
+                );
+            });
+        }
+    }
+
+    _accumulateRoom(roomId, category, data) {
+        return;
+    }
+
+    /**
+     * Return everything under the 'rooms' key from a /sync response which
+     * accurately represents all room data.
+     * @return {Object} A JSON object which has the same API shape as /sync.
+     */
+    getJSON() {
+        const data = {
+            join: {},
+            invite: {},
+            leave: {},
+        };
+        Object.keys(this.rooms).forEach((roomId) => {
+            switch (this.rooms[roomId].category) {
+                case "join":
+                    data.join[roomId] = this.rooms[roomId].data;
+                    break;
+                case "invite":
+                    data.invite[roomId] = this.rooms[roomId].data;
+                    break;
+                case "leave":
+                    data.leave[roomId] = this.rooms[roomId].data;
+                    break;
+            }
+        });
+        return data;
+    }
+}
+
+module.exports = SyncAccumulator;