diff --git a/requests/cookies.py b/requests/cookies.py
index a4140adf..c465f552 100644
--- a/requests/cookies.py
+++ b/requests/cookies.py
@@ -421,3 +421,25 @@ def cookiejar_from_dict(cookie_dict, cookiejar=None, overwrite=True):
                 cookiejar.set_cookie(create_cookie(name, cookie_dict[name]))
 
     return cookiejar
+
+
+def merge_cookies(cookiejar, cookies):
+    """Add cookies to cookiejar and returns a merged CookieJar.
+
+    :param cookiejar: CookieJar object to add the cookies to.
+    :param cookies: Dictionary or CookieJar object to be added.
+    """
+    if not isinstance(cookiejar, cookielib.CookieJar):
+        raise ValueError('You can only merge into CookieJar')
+    
+    if isinstance(cookies, dict):
+        cookiejar = cookiejar_from_dict(
+            cookies, cookiejar=cookiejar, overwrite=False)
+    elif isinstance(cookies, cookielib.CookieJar):
+        try:
+            cookiejar.update(cookies)
+        except AttributeError:
+            for cookie_in_jar in cookies:
+                cookiejar.set_cookie(cookie_in_jar)
+
+    return cookiejar
diff --git a/requests/sessions.py b/requests/sessions.py
index 9c95b58d..ab60aa8e 100644
--- a/requests/sessions.py
+++ b/requests/sessions.py
@@ -13,7 +13,8 @@ from collections import Mapping
 from datetime import datetime
 
 from .compat import cookielib, OrderedDict, urljoin, urlparse, builtin_str
-from .cookies import cookiejar_from_dict, extract_cookies_to_jar, RequestsCookieJar
+from .cookies import (
+    cookiejar_from_dict, extract_cookies_to_jar, RequestsCookieJar, merge_cookies)
 from .models import Request, PreparedRequest
 from .hooks import default_hooks, dispatch_hook
 from .utils import to_key_val_list, default_headers
@@ -245,9 +246,8 @@ class Session(SessionRedirectMixin):
             cookies = cookiejar_from_dict(cookies)
 
         # Merge with session cookies
-        merged_cookies = RequestsCookieJar()
-        merged_cookies.update(self.cookies)
-        merged_cookies.update(cookies)
+        merged_cookies = merge_cookies(
+            merge_cookies(RequestsCookieJar(), self.cookies), cookies)
 
 
         # Set environment's basic authentication if not explicitly set.
@@ -330,7 +330,7 @@ class Session(SessionRedirectMixin):
         prep = self.prepare_request(req)
 
         # Add param cookies to session cookies
-        self.cookies = cookiejar_from_dict(cookies, cookiejar=self.cookies, overwrite=False)
+        self.cookies = merge_cookies(self.cookies, cookies)
 
         proxies = proxies or {}
 
diff --git a/test_requests.py b/test_requests.py
index 3b673956..539de764 100755
--- a/test_requests.py
+++ b/test_requests.py
@@ -187,6 +187,14 @@ class RequestsTestCase(unittest.TestCase):
         assert r.json()['cookies']['foo'] == 'bar'
         # Make sure the session cj is still the custom one
         assert s.cookies is cj
+    
+    def test_param_cookiejar_works(self):
+        cj = cookielib.CookieJar()
+        cookiejar_from_dict({'foo' : 'bar'}, cj)
+        s = requests.session()
+        r = s.get(httpbin('cookies'), cookies=cj)
+        # Make sure the cookie was sent
+        assert r.json()['cookies']['foo'] == 'bar'
 
     def test_requests_in_history_are_not_overridden(self):
         resp = requests.get(httpbin('redirect/3'))