diff --git a/requests/models.py b/requests/models.py
index a7009eb5..59f8ad44 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -195,6 +195,8 @@ class Request(RequestHooksMixin):
         p.prepare_headers(self.headers)
         p.prepare_cookies(self.cookies)
         p.prepare_body(self.data, self.files)
+        # Note that prepare_auth most be last to enable authentication schemes
+        # such as OAuth to work on a fully prepared request.
         p.prepare_auth(self.auth)
 
         return p