diff --git a/pubsub/google/cloud/pubsub/_http.py b/pubsub/google/cloud/pubsub/_http.py
index 6aef4edd6437..635c43bdaab5 100644
--- a/pubsub/google/cloud/pubsub/_http.py
+++ b/pubsub/google/cloud/pubsub/_http.py
@@ -15,6 +15,7 @@
 """Create / interact with Google Cloud Pub/Sub connections."""
 
 import base64
+import copy
 import functools
 import os
 
@@ -203,9 +204,10 @@ def topic_publish(self, topic_path, messages):
         :rtype: list of string
         :returns: list of opaque IDs for published messages.
         """
-        _transform_messages_base64(messages, _base64_unicode)
+        messages_to_send = copy.deepcopy(messages)
+        _transform_messages_base64(messages_to_send, _base64_unicode)
         conn = self._connection
-        data = {'messages': messages}
+        data = {'messages': messages_to_send}
         response = conn.api_request(
             method='POST', path='/%s:publish' % (topic_path,), data=data)
         return response['messageIds']
diff --git a/pubsub/unit_tests/test__http.py b/pubsub/unit_tests/test__http.py
index 3401d0c5869e..3a0281a03eb2 100644
--- a/pubsub/unit_tests/test__http.py
+++ b/pubsub/unit_tests/test__http.py
@@ -284,10 +284,32 @@ def test_topic_publish_hit(self):
         msg_data = connection._called_with['data']['messages'][0]['data']
         self.assertEqual(msg_data, B64_PAYLOAD)
 
+    def test_topic_publish_twice(self):
+        import base64
+
+        PAYLOAD = b'This is the message text'
+        B64_PAYLOAD = base64.b64encode(PAYLOAD).decode('ascii')
+        MESSAGE = {'data': PAYLOAD, 'attributes': {}}
+        RETURNED = {'messageIds': []}
+        connection = _Connection(RETURNED, RETURNED)
+        client = _Client(connection, self.PROJECT)
+        api = self._make_one(client)
+
+        api.topic_publish(self.TOPIC_PATH, [MESSAGE])
+        api.topic_publish(self.TOPIC_PATH, [MESSAGE])
+
+        messages = connection._called_with['data']['messages']
+        self.assertEqual(len(messages), 1)
+        self.assertEqual(messages[0]['data'], B64_PAYLOAD)
+
     def test_topic_publish_miss(self):
+        import base64
         from google.cloud.exceptions import NotFound
+
         PAYLOAD = b'This is the message text'
+        B64_PAYLOAD = base64.b64encode(PAYLOAD).decode('ascii')
         MESSAGE = {'data': PAYLOAD, 'attributes': {}}
+        B64MSG = {'data': B64_PAYLOAD, 'attributes': {}}
         connection = _Connection()
         client = _Client(connection, self.PROJECT)
         api = self._make_one(client)
@@ -299,7 +321,7 @@ def test_topic_publish_miss(self):
         path = '/%s:publish' % (self.TOPIC_PATH,)
         self.assertEqual(connection._called_with['path'], path)
         self.assertEqual(connection._called_with['data'],
-                         {'messages': [MESSAGE]})
+                         {'messages': [B64MSG]})
 
     def test_topic_list_subscriptions_no_paging(self):
         from google.cloud.pubsub.topic import Topic