diff --git a/moto/core/models.py b/moto/core/models.py
index 1b0f2d0dd..896f9ac4a 100644
--- a/moto/core/models.py
+++ b/moto/core/models.py
@@ -12,6 +12,7 @@ from collections import defaultdict
 from botocore.handlers import BUILTIN_HANDLERS
 from botocore.awsrequest import AWSResponse
 
+import mock
 from moto import settings
 import responses
 from moto.packages.httpretty import HTTPretty
@@ -22,11 +23,6 @@ from .utils import (
 )
 
 
-# "Mock" the AWS credentials as they can't be mocked in Botocore currently
-os.environ.setdefault("AWS_ACCESS_KEY_ID", "foobar_key")
-os.environ.setdefault("AWS_SECRET_ACCESS_KEY", "foobar_secret")
-
-
 class BaseMockAWS(object):
     nested_count = 0
 
@@ -42,6 +38,10 @@ class BaseMockAWS(object):
         self.backends_for_urls.update(self.backends)
         self.backends_for_urls.update(default_backends)
 
+        # "Mock" the AWS credentials as they can't be mocked in Botocore currently
+        FAKE_KEYS = {"AWS_ACCESS_KEY_ID": "foobar_key", "AWS_SECRET_ACCESS_KEY": "foobar_secret"}
+        self.env_variables_mocks = mock.patch.dict(os.environ, FAKE_KEYS)
+
         if self.__class__.nested_count == 0:
             self.reset()
 
@@ -58,6 +58,8 @@ class BaseMockAWS(object):
         self.stop()
 
     def start(self, reset=True):
+        self.env_variables_mocks.start()
+
         self.__class__.nested_count += 1
         if reset:
             for backend in self.backends.values():
@@ -66,6 +68,7 @@ class BaseMockAWS(object):
         self.enable_patching()
 
     def stop(self):
+        self.env_variables_mocks.stop()
         self.__class__.nested_count -= 1
 
         if self.__class__.nested_count < 0: