diff --git a/pylearn2/expr/nnet.py b/pylearn2/expr/nnet.py
index 3d36cc71b6..c816e5b810 100644
--- a/pylearn2/expr/nnet.py
+++ b/pylearn2/expr/nnet.py
@@ -114,6 +114,46 @@ def arg_of_softmax(Y_hat):
     return z
 
 
+def arg_of_sigmoid(Y_hat):
+    """
+    Given the output of a call to theano.tensor.nnet.sigmoid,
+    returns the argument to the sigmoid (by tracing the Theano
+    graph).
+
+    Parameters
+    ----------
+    Y_hat : Variable
+        T.nnet.sigmoid(Z)
+
+    Returns
+    -------
+    Z : Variable
+        The variable that was passed to T.nnet.sigmoid to create `Y_hat`.
+        Raises an error if `Y_hat` is not actually the output of a theano
+        sigmoid.
+    """
+    assert hasattr(Y_hat, 'owner')
+    owner = Y_hat.owner
+    assert owner is not None
+    op = owner.op
+    if isinstance(op, Print):
+        assert len(owner.inputs) == 1
+        Y_hat, = owner.inputs
+        owner = Y_hat.owner
+        op = owner.op
+    success = False
+    if isinstance(op, T.Elemwise):
+        if isinstance(op.scalar_op, T.nnet.sigm.ScalarSigmoid):
+            success = True
+    if not success:
+        raise TypeError("Expected Y_hat to be the output of a sigmoid, "
+                        "but it appears to be the output of " + str(op) +
+                        " of type " + str(type(op)))
+    z, = owner.inputs
+    assert z.ndim == 2
+    return z
+
+
 def kl(Y, Y_hat, batch_axis):
     """
     Warning: This function expects a sigmoid nonlinearity in the
@@ -323,4 +363,4 @@ def compute_f1(precision, recall):
     """
     f1 = (2. * precision * recall /
           T.maximum(1, precision + recall))
-    return f1
\ No newline at end of file
+    return f1
diff --git a/pylearn2/expr/tests/test_nnet.py b/pylearn2/expr/tests/test_nnet.py
index 6de0f1c3b7..2d8277770b 100644
--- a/pylearn2/expr/tests/test_nnet.py
+++ b/pylearn2/expr/tests/test_nnet.py
@@ -14,12 +14,13 @@
 from theano import tensor as T
 
 from pylearn2.models.mlp import MLP, Sigmoid
+from pylearn2.expr.nnet import arg_of_sigmoid
 from pylearn2.expr.nnet import pseudoinverse_softmax_numpy
 from pylearn2.expr.nnet import softmax_numpy
 from pylearn2.expr.nnet import softmax_ratio
 from pylearn2.expr.nnet import compute_recall
 from pylearn2.expr.nnet import kl
-from pylearn2.expr.nnet import elemwise_kl 
+from pylearn2.expr.nnet import elemwise_kl
 from pylearn2.utils import sharedX
 
 
@@ -83,7 +84,7 @@ def test_kl():
     """
     init_mode = theano.config.compute_test_value
     theano.config.compute_test_value = 'raise'
-    
+
     try:
         mlp = MLP(layers=[Sigmoid(dim=10, layer_name='Y', irange=0.1)],
                   nvis=10)
@@ -101,7 +102,7 @@ def test_kl():
         np.testing.assert_raises(ValueError, kl, Y, Y_hat, 1)
         Y.tag.test_value[2][3] = -0.1
         np.testing.assert_raises(ValueError, kl, Y, Y_hat, 1)
-    
+
     finally:
         theano.config.compute_test_value = init_mode
 
@@ -112,10 +113,10 @@ def test_elemwise_kl():
     input.
     """
     init_mode = theano.config.compute_test_value
-    theano.config.compute_test_value = 'raise' 
-    
+    theano.config.compute_test_value = 'raise'
+
     try:
-        mlp = MLP(layers=[Sigmoid(dim=10, layer_name='Y', irange=0.1)], 
+        mlp = MLP(layers=[Sigmoid(dim=10, layer_name='Y', irange=0.1)],
                   nvis=10)
         X = mlp.get_input_space().make_theano_batch()
         Y = mlp.get_output_space().make_theano_batch()
@@ -131,8 +132,29 @@ def test_elemwise_kl():
         np.testing.assert_raises(ValueError, elemwise_kl, Y, Y_hat)
         Y.tag.test_value[2][3] = -0.1
         np.testing.assert_raises(ValueError, elemwise_kl, Y, Y_hat)
-    
+
     finally:
         theano.config.compute_test_value = init_mode
 
-   
+def test_arg_of_sigmoid_good():
+    """
+    Tests that arg_of_sigmoid works when given a good input.
+    """
+
+    X = T.matrix()
+    Y = T.nnet.sigmoid(X)
+    Z = arg_of_sigmoid(Y)
+    assert X is Z
+
+def test_arg_of_sigmoid_bad():
+    """
+    Tests that arg_of_sigmoid raises an error when given a bad input.
+    """
+
+    X = T.matrix()
+    Y = T.nnet.softmax(X)
+    try:
+        Z = arg_of_sigmoid(Y)
+    except TypeError:
+        return
+    assert False # Should have failed