diff --git a/SimPEG/PropMaps.py b/SimPEG/PropMaps.py
index d7bd2ba2..bddd3df7 100644
--- a/SimPEG/PropMaps.py
+++ b/SimPEG/PropMaps.py
@@ -238,3 +238,6 @@ class PropMap(object):
     def __call__(self, vec):
         return self.PropModel(self, vec)
 
+    def __contains__(self, val):
+        activeMaps = [name for name in self._properties if getattr(self, '%sMap'%name) is not None]
+        return val in activeMaps
diff --git a/SimPEG/Tests/test_PropMaps.py b/SimPEG/Tests/test_PropMaps.py
index c2726d71..6861f09b 100644
--- a/SimPEG/Tests/test_PropMaps.py
+++ b/SimPEG/Tests/test_PropMaps.py
@@ -36,6 +36,10 @@ class TestPropMaps(unittest.TestCase):
             assert PM.muMap is None
             assert PM.muIndex is None
 
+            assert 'sigma' in PM
+            assert 'mu' not in PM
+            assert 'mui' not in PM
+
             m = PM(np.r_[1.,2,3])
             assert m.mu == mu_0
             assert m.muModel is None
@@ -67,6 +71,10 @@ class TestPropMaps(unittest.TestCase):
 
         assert pm.nP == 6
 
+        assert 'sigma' in PM
+        assert 'mu' in PM
+        assert 'mui' not in PM
+
         assert np.all(pm.sigmaModel == [1.,2,3])
         assert np.all(pm.sigma == np.exp([1.,2,3]))
         assert np.all(pm.muModel == [4.,5,6])
@@ -83,6 +91,10 @@ class TestPropMaps(unittest.TestCase):
 
         assert pm.nP == 3
 
+        assert 'sigma' in PM
+        assert 'mu' in PM
+        assert 'mui' not in PM
+
         assert np.all(pm.sigmaModel == [1,2,3])
         assert np.all(pm.sigma == np.exp([1,2,3]))
         assert np.all(pm.muModel == [1,2,3])
@@ -103,6 +115,11 @@ class TestPropMaps(unittest.TestCase):
         assert pm.sigmaDeriv is not None
         assert pm.rhoDeriv is not None
 
+        assert 'sigma' in PM
+        assert 'rho' not in PM
+        assert 'mu' not in PM
+        assert 'mui' not in PM
+
         assert pm.mu == mu_0
         assert pm.mui == 1.0/mu_0
         assert pm.muMap is None
@@ -121,6 +138,11 @@ class TestPropMaps(unittest.TestCase):
         assert pm.sigmaDeriv is not None
         assert pm.rhoDeriv is not None
 
+        assert 'sigma' not in PM
+        assert 'rho' in PM
+        assert 'mu' not in PM
+        assert 'mui' not in PM
+
         self.assertRaises(AssertionError, MyReciprocalPropMap, [('rho', iMap), ('sigma', iMap)])
         self.assertRaises(AssertionError, MyReciprocalPropMap, [('sigma', iMap), ('rho', iMap)])