diff --git a/requests/cookies.py b/requests/cookies.py
index 6969fe5c..1fbc934c 100644
--- a/requests/cookies.py
+++ b/requests/cookies.py
@@ -6,6 +6,7 @@ Compatibility code to be able to use `cookielib.CookieJar` with requests.
 requests.utils imports from here, so be careful with imports.
 """
 
+import copy
 import time
 import collections
 from .compat import cookielib, urlparse, urlunparse, Morsel
@@ -302,7 +303,7 @@ class RequestsCookieJar(cookielib.CookieJar, collections.MutableMapping):
         """Updates this jar with cookies from another CookieJar or dict-like"""
         if isinstance(other, cookielib.CookieJar):
             for cookie in other:
-                self.set_cookie(cookie)
+                self.set_cookie(copy.copy(cookie))
         else:
             super(RequestsCookieJar, self).update(other)
 
@@ -359,6 +360,21 @@ class RequestsCookieJar(cookielib.CookieJar, collections.MutableMapping):
         return new_cj
 
 
+def _copy_cookie_jar(jar):
+    if jar is None:
+        return None
+
+    if hasattr(jar, 'copy'):
+        # We're dealing with an instane of RequestsCookieJar
+        return jar.copy()
+    # We're dealing with a generic CookieJar instance
+    new_jar = copy.copy(jar)
+    new_jar.clear()
+    for cookie in jar:
+        new_jar.set_cookie(copy.copy(cookie))
+    return new_jar
+
+
 def create_cookie(name, value, **kwargs):
     """Make a cookie from underspecified parameters.
 
diff --git a/requests/models.py b/requests/models.py
index cb153aa5..12304adb 100644
--- a/requests/models.py
+++ b/requests/models.py
@@ -15,7 +15,7 @@ from .hooks import default_hooks
 from .structures import CaseInsensitiveDict
 
 from .auth import HTTPBasicAuth
-from .cookies import cookiejar_from_dict, get_cookie_header
+from .cookies import cookiejar_from_dict, get_cookie_header, _copy_cookie_jar
 from .packages.urllib3.fields import RequestField
 from .packages.urllib3.filepost import encode_multipart_formdata
 from .packages.urllib3.util import parse_url
@@ -320,7 +320,7 @@ class PreparedRequest(RequestEncodingMixin, RequestHooksMixin):
         p.method = self.method
         p.url = self.url
         p.headers = self.headers.copy() if self.headers is not None else None
-        p._cookies = self._cookies.copy() if self._cookies is not None else None
+        p._cookies = _copy_cookie_jar(self._cookies)
         p.body = self.body
         p.hooks = self.hooks
         return p