diff --git a/js/libtextsecure.js b/js/libtextsecure.js
index 749d2e80..c13c4b5e 100644
--- a/js/libtextsecure.js
+++ b/js/libtextsecure.js
@@ -39634,6 +39634,7 @@ textsecure.MessageReceiver.prototype = {
  */
 function MessageSender(url, username, password) {
     this.server = new TextSecureServer(url, username, password);
+    this.pendingMessages = {};
 }
 
 MessageSender.prototype = {
@@ -39736,7 +39737,7 @@ MessageSender.prototype = {
                 return textsecure.storage.devices.getDeviceObjectsForNumber(number).then(function(devicesForNumber) {
                     if (devicesForNumber.length == 0)
                         return registerError(number, "Got empty device list when loading device keys", null);
-                    doSendMessage(number, devicesForNumber, recurse);
+                    return doSendMessage(number, devicesForNumber, recurse);
                 });
             }
         };
@@ -39795,9 +39796,9 @@ MessageSender.prototype = {
                         }));
                     }
 
-                    p.then(function() {
+                    return p.then(function() {
                         var resetDevices = ((error.code == 410) ? error.response.staleDevices : error.response.missingDevices);
-                        getKeysForNumber(number, resetDevices)
+                        return getKeysForNumber(number, resetDevices)
                             .then(reloadDevicesAndSend(number, (error.code == 409)))
                             .catch(function(error) {
                                 registerError(number, "Failed to reload device keys", error);
@@ -39809,8 +39810,8 @@ MessageSender.prototype = {
             });
         }.bind(this);
 
-        numbers.forEach(function(number) {
-            textsecure.storage.devices.getDeviceObjectsForNumber(number).then(function(devicesForNumber) {
+        function sendToNumber(number) {
+            return textsecure.storage.devices.getDeviceObjectsForNumber(number).then(function(devicesForNumber) {
                 return Promise.all(devicesForNumber.map(function(device) {
                     return textsecure.protocol_wrapper.hasOpenSession(device.encodedNumber).then(function(result) {
                         if (!result)
@@ -39819,17 +39820,31 @@ MessageSender.prototype = {
                 })).then(function() {
                     return textsecure.storage.devices.getDeviceObjectsForNumber(number).then(function(devicesForNumber) {
                         if (devicesForNumber.length == 0) {
-                            getKeysForNumber(number, [1])
+                            return getKeysForNumber(number, [1])
                                 .then(reloadDevicesAndSend(number, true))
                                 .catch(function(error) {
                                     registerError(number, "Failed to retreive new device keys for number " + number, error);
                                 });
                         } else
-                            doSendMessage(number, devicesForNumber, true);
+                            return doSendMessage(number, devicesForNumber, true);
                     });
                 });
             });
-        });
+        }
+
+        numbers.forEach(function(number) {
+            var sendPrevious = this.pendingMessages[number] || Promise.resolve();
+            var sendCurrent = this.pendingMessages[number] = sendPrevious.then(function() {
+                return sendToNumber(number);
+            }).catch(function() {
+                return sendToNumber(number);
+            });
+            sendCurrent.then(function() {
+                if (this.pendingMessages[number] === sendCurrent) {
+                    delete this.pendingMessages[number];
+                }
+            }.bind(this));
+        }.bind(this));
     },
 
     sendIndividualProto: function(number, proto, timestamp) {
diff --git a/libtextsecure/sendmessage.js b/libtextsecure/sendmessage.js
index 514a2c12..9ef7eda2 100644
--- a/libtextsecure/sendmessage.js
+++ b/libtextsecure/sendmessage.js
@@ -3,6 +3,7 @@
  */
 function MessageSender(url, username, password) {
     this.server = new TextSecureServer(url, username, password);
+    this.pendingMessages = {};
 }
 
 MessageSender.prototype = {
@@ -105,7 +106,7 @@ MessageSender.prototype = {
                 return textsecure.storage.devices.getDeviceObjectsForNumber(number).then(function(devicesForNumber) {
                     if (devicesForNumber.length == 0)
                         return registerError(number, "Got empty device list when loading device keys", null);
-                    doSendMessage(number, devicesForNumber, recurse);
+                    return doSendMessage(number, devicesForNumber, recurse);
                 });
             }
         };
@@ -164,9 +165,9 @@ MessageSender.prototype = {
                         }));
                     }
 
-                    p.then(function() {
+                    return p.then(function() {
                         var resetDevices = ((error.code == 410) ? error.response.staleDevices : error.response.missingDevices);
-                        getKeysForNumber(number, resetDevices)
+                        return getKeysForNumber(number, resetDevices)
                             .then(reloadDevicesAndSend(number, (error.code == 409)))
                             .catch(function(error) {
                                 registerError(number, "Failed to reload device keys", error);
@@ -178,8 +179,8 @@ MessageSender.prototype = {
             });
         }.bind(this);
 
-        numbers.forEach(function(number) {
-            textsecure.storage.devices.getDeviceObjectsForNumber(number).then(function(devicesForNumber) {
+        function sendToNumber(number) {
+            return textsecure.storage.devices.getDeviceObjectsForNumber(number).then(function(devicesForNumber) {
                 return Promise.all(devicesForNumber.map(function(device) {
                     return textsecure.protocol_wrapper.hasOpenSession(device.encodedNumber).then(function(result) {
                         if (!result)
@@ -188,17 +189,31 @@ MessageSender.prototype = {
                 })).then(function() {
                     return textsecure.storage.devices.getDeviceObjectsForNumber(number).then(function(devicesForNumber) {
                         if (devicesForNumber.length == 0) {
-                            getKeysForNumber(number, [1])
+                            return getKeysForNumber(number, [1])
                                 .then(reloadDevicesAndSend(number, true))
                                 .catch(function(error) {
                                     registerError(number, "Failed to retreive new device keys for number " + number, error);
                                 });
                         } else
-                            doSendMessage(number, devicesForNumber, true);
+                            return doSendMessage(number, devicesForNumber, true);
                     });
                 });
             });
-        });
+        }
+
+        numbers.forEach(function(number) {
+            var sendPrevious = this.pendingMessages[number] || Promise.resolve();
+            var sendCurrent = this.pendingMessages[number] = sendPrevious.then(function() {
+                return sendToNumber(number);
+            }).catch(function() {
+                return sendToNumber(number);
+            });
+            sendCurrent.then(function() {
+                if (this.pendingMessages[number] === sendCurrent) {
+                    delete this.pendingMessages[number];
+                }
+            }.bind(this));
+        }.bind(this));
     },
 
     sendIndividualProto: function(number, proto, timestamp) {