diff --git a/src/Lifecycle.js b/src/Lifecycle.js
index 6403cd6d4d..ee82a8890a 100644
--- a/src/Lifecycle.js
+++ b/src/Lifecycle.js
@@ -134,6 +134,7 @@ function _loginWithToken(queryParams, defaultDeviceDisplayName) {
         console.log("Logged in with token");
         setLoggedIn({
             userId: data.user_id,
+            deviceId: data.device_id,
             accessToken: data.access_token,
             homeserverUrl: queryParams.homeserver,
             identityServerUrl: queryParams.identityServer,
@@ -164,6 +165,7 @@ function _registerAsGuest(hsUrl, isUrl, defaultDeviceDisplayName) {
         console.log("Registered as guest: %s", creds.user_id);
         setLoggedIn({
             userId: creds.user_id,
+            deviceId: creds.device_id,
             accessToken: creds.access_token,
             homeserverUrl: hsUrl,
             identityServerUrl: isUrl,
@@ -183,6 +185,7 @@ function _restoreFromLocalStorage() {
     const is_url = localStorage.getItem("mx_is_url") || 'https://matrix.org';
     const access_token = localStorage.getItem("mx_access_token");
     const user_id = localStorage.getItem("mx_user_id");
+    const device_id = localStorage.getItem("mx_device_id");
 
     let is_guest;
     if (localStorage.getItem("mx_is_guest") !== null) {
@@ -196,6 +199,7 @@ function _restoreFromLocalStorage() {
         console.log("Restoring session for %s", user_id);
         setLoggedIn({
             userId: user_id,
+            deviceId: device_id,
             accessToken: access_token,
             homeserverUrl: hs_url,
             identityServerUrl: is_url,
@@ -223,10 +227,19 @@ export function setLoggedIn(credentials) {
         try {
             localStorage.setItem("mx_hs_url", credentials.homeserverUrl);
             localStorage.setItem("mx_is_url", credentials.identityServerUrl);
-
             localStorage.setItem("mx_user_id", credentials.userId);
             localStorage.setItem("mx_access_token", credentials.accessToken);
             localStorage.setItem("mx_is_guest", JSON.stringify(credentials.guest));
+
+            // if we didn't get a deviceId from the login, leave mx_device_id unset,
+            // rather than setting it to "undefined".
+            //
+            // (in this case MatrixClient doesn't bother with the crypto stuff
+            // - that's fine for us).
+            if (credentials.deviceId) {
+                localStorage.setItem("mx_device_id", credentials.deviceId);
+            }
+
             console.log("Session persisted for %s", credentials.userId);
         } catch (e) {
             console.warn("Error using local storage: can't persist session!", e);
diff --git a/src/MatrixClientPeg.js b/src/MatrixClientPeg.js
index e22990a850..190181c875 100644
--- a/src/MatrixClientPeg.js
+++ b/src/MatrixClientPeg.js
@@ -21,21 +21,11 @@ import utils from 'matrix-js-sdk/lib/utils';
 
 const localStorage = window.localStorage;
 
-function deviceId() {
-    // XXX: is Math.random()'s deterministicity a problem here?
-    var id = Math.floor(Math.random()*16777215).toString(16);
-    id = "W" + "000000".substring(id.length) + id;
-    if (localStorage) {
-        id = localStorage.getItem("mx_device_id") || id;
-        localStorage.setItem("mx_device_id", id);
-    }
-    return id;
-}
-
 interface MatrixClientCreds {
     homeserverUrl: string,
     identityServerUrl: string,
     userId: string,
+    deviceId: string,
     accessToken: string,
     guest: boolean,
 }
@@ -87,6 +77,7 @@ class MatrixClientPeg {
             homeserverUrl: this.matrixClient.baseUrl,
             identityServerUrl: this.matrixClient.idBaseUrl,
             userId: this.matrixClient.credentials.userId,
+            deviceId: this.matrixClient.getDeviceId(),
             accessToken: this.matrixClient.getAccessToken(),
             guest: this.matrixClient.isGuest(),
         };
@@ -98,12 +89,12 @@ class MatrixClientPeg {
             idBaseUrl: creds.identityServerUrl,
             accessToken: creds.accessToken,
             userId: creds.userId,
+            deviceId: creds.deviceId,
             timelineSupport: true,
         };
 
         if (localStorage) {
             opts.sessionStore = new Matrix.WebStorageSessionStore(localStorage);
-            opts.deviceId = deviceId();
         }
 
         this.matrixClient = Matrix.createClient(opts);
diff --git a/src/Signup.js b/src/Signup.js
index 9eb19d0702..1ac92f3218 100644
--- a/src/Signup.js
+++ b/src/Signup.js
@@ -336,6 +336,7 @@ class Login extends Signup {
         }).then((creds) => {
             return {
                 userId: creds.user_id,
+                deviceId: creds.device_id,
                 accessToken: creds.access_token,
                 homeserverUrl: this._hsUrl,
                 identityServerUrl: this._isUrl,
@@ -371,6 +372,7 @@ class Login extends Signup {
                 homeserverUrl: self._hsUrl,
                 identityServerUrl: self._isUrl,
                 userId: data.user_id,
+                deviceId: data.device_id,
                 accessToken: data.access_token
             });
         }, function(error) {
@@ -394,6 +396,7 @@ class Login extends Signup {
                             homeserverUrl: self._fallbackHsUrl,
                             identityServerUrl: self._isUrl,
                             userId: data.user_id,
+                            deviceId: data.device_id,
                             accessToken: data.access_token
                         });
                     }, function(fallback_error) {
diff --git a/src/components/structures/login/Registration.js b/src/components/structures/login/Registration.js
index 84bfbe9834..0f1b6d331f 100644
--- a/src/components/structures/login/Registration.js
+++ b/src/components/structures/login/Registration.js
@@ -159,6 +159,7 @@ module.exports = React.createClass({
             }
             self.props.onLoggedIn({
                 userId: response.user_id,
+                deviceId: response.device_id,
                 homeserverUrl: self.registerLogic.getHomeserverUrl(),
                 identityServerUrl: self.registerLogic.getIdentityServerUrl(),
                 accessToken: response.access_token
@@ -284,7 +285,7 @@ module.exports = React.createClass({
 
         var returnToAppJsx;
         if (this.props.onCancelClick) {
-            returnToAppJsx = 
+            returnToAppJsx =
                 <a className="mx_Login_create" onClick={this.props.onCancelClick} href="#">
                     Return to app
                 </a>