diff --git a/lib/pubsub/subscription.js b/lib/pubsub/subscription.js
index fe110e4a51a..97d80ea6927 100644
--- a/lib/pubsub/subscription.js
+++ b/lib/pubsub/subscription.js
@@ -102,7 +102,8 @@ var util = require('../common/util.js');
  * // Register a listener for `message` events.
  * function onMessage(message) {
  *   // Called every time a message is received.
- *   // message.id = ID used to acknowledge its receival.
+ *   // message.id = ID of the message.
+ *   // message.ackId = ID used to acknowledge the message receival.
  *   // message.data = Contents of the message.
  * }
  * subscription.on('message', onMessage);
@@ -243,24 +244,24 @@ Subscription.prototype.startPulling_ = function() {
 
 /**
  * Acknowledge to the backend that the message was retrieved. You must provide
- * either a single ID or an array of IDs.
+ * either a single ackId or an array of ackIds.
  *
- * @throws {Error} If at least one id is not provided.
+ * @throws {Error} If at least one ackId is not provided.
  *
- * @param {string|string[]} ids - An ID or array of message IDs.
+ * @param {string|string[]} ackIds - An ackId or array of ackIds.
  * @param {function=} callback - The callback function.
  *
  * @example
  * subscription.ack('ePHEESyhuE8e...', function(err, apiResponse) {});
  */
-Subscription.prototype.ack = function(ids, callback) {
-  if (!ids || ids.length === 0) {
+Subscription.prototype.ack = function(ackIds, callback) {
+  if (!ackIds || ackIds.length === 0) {
     throw new Error(
         'At least one ID must be specified before it can be acknowledged.');
   }
-  ids = util.arrayize(ids);
+  ackIds = util.arrayize(ackIds);
   var body = {
-    ackIds: ids
+    ackIds: ackIds
   };
   var path = this.name + ':acknowledge';
   this.makeReq_('POST', path, null, body, callback || util.noop);