diff --git a/requests/models.py b/requests/models.py
index 9c624d3c..4bcbc548 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -81,7 +81,7 @@ class RequestEncodingMixin(object):
         """
 
         if isinstance(data, (str, bytes)):
-            return to_native_string(data)
+            return data
         elif hasattr(data, 'read'):
             return data
         elif hasattr(data, '__iter__'):
@@ -385,6 +385,9 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
             if isinstance(fragment, str):
                 fragment = fragment.encode('utf-8')
 
+        if isinstance(params, (str, bytes)):
+            params = to_native_string(params)
+
         enc_params = self._encode_params(params)
         if enc_params:
             if query:
diff --git a/test_requests.py b/test_requests.py
index 137edc08..07952418 100755
--- a/test_requests.py
+++ b/test_requests.py
@@ -157,6 +157,11 @@ class TestRequests(object):
                                    params=b'test=foo').prepare()
         assert request.url == 'http://example.com/?test=foo'
 
+    def test_binary_put(self):
+        request = requests.Request('PUT', 'http://example.com',
+                                   data=u"ööö".encode("utf-8")).prepare()
+        assert isinstance(request.body, bytes)
+
     def test_mixed_case_scheme_acceptable(self, httpbin):
         s = requests.Session()
         s.proxies = getproxies()