diff --git a/interfaces/cython/cantera/_cantera.pxd b/interfaces/cython/cantera/_cantera.pxd
index b7cdf9ce8..d95535ec0 100644
--- a/interfaces/cython/cantera/_cantera.pxd
+++ b/interfaces/cython/cantera/_cantera.pxd
@@ -86,11 +86,13 @@ cdef extern from "cantera/thermo/ThermoPhase.h" namespace "Cantera":
         # composition
         void setMassFractionsByName(string) except +
         void setMassFractionsByName(stdmap[string,double]&) except +
+        void setMassFractions_NoNorm(double*) except +
         double massFraction(size_t) except +
         double massFraction(string) except +
 
         void setMoleFractionsByName(string) except +
         void setMoleFractionsByName(stdmap[string,double]&) except +
+        void setMoleFractions_NoNorm(double*) except +
         void getMoleFractions(double*) except +
         double moleFraction(size_t) except +
         double moleFraction(string) except +
diff --git a/interfaces/cython/cantera/test/test_thermo.py b/interfaces/cython/cantera/test/test_thermo.py
index 7a911c009..3f8d614b3 100644
--- a/interfaces/cython/cantera/test/test_thermo.py
+++ b/interfaces/cython/cantera/test/test_thermo.py
@@ -94,6 +94,29 @@ class TestThermoPhase(utilities.CanteraTest):
         self.assertNear(Y[0], 0.25)
         self.assertNear(Y[3], 0.75)
 
+    def test_setCompositionNoNorm(self):
+        X = np.zeros(self.phase.n_species)
+        X[2] = 1.0
+        X[0] = 0.01
+        self.phase.set_unnormalized_mole_fractions(X)
+        self.assertArrayNear(self.phase.X, X)
+        self.assertNear(sum(X), 1.01)
+
+        Y = np.zeros(self.phase.n_species)
+        Y[2] = 1.0
+        Y[0] = 0.01
+        self.phase.set_unnormalized_mass_fractions(Y)
+        self.assertArrayNear(self.phase.Y, Y)
+        self.assertNear(sum(Y), 1.01)
+
+    def test_setCompositionNoNormBad(self):
+        X = np.zeros(self.phase.n_species - 1)
+        with self.assertRaises(ValueError):
+            self.phase.set_unnormalized_mole_fractions(X)
+
+        with self.assertRaises(ValueError):
+            self.phase.set_unnormalized_mass_fractions([1,2,3])
+
     @unittest.expectedFailure
     def test_setCompositionDict_bad1(self):
         # Non-existent species should raise an exception
diff --git a/interfaces/cython/cantera/thermo.pyx b/interfaces/cython/cantera/thermo.pyx
index a317a7cfe..39a9df640 100644
--- a/interfaces/cython/cantera/thermo.pyx
+++ b/interfaces/cython/cantera/thermo.pyx
@@ -317,6 +317,32 @@ cdef class ThermoPhase(_SolutionBase):
         def __set__(self, C):
             self._setArray1(thermo_setConcentrations, C)
 
+    def set_unnormalized_mass_fractions(self, Y):
+        """
+        Set the mass fractions without normalizing to force sum(Y) == 1.0.
+        Useful primarily when calculating derivatives with respect to Y[k] by
+        finite difference.
+        """
+        cdef np.ndarray[np.double_t, ndim=1] data
+        if len(Y) == self.n_species:
+            data = np.ascontiguousarray(Y, dtype=np.double)
+        else:
+            raise ValueError("Array has incorrect length")
+        self.thermo.setMassFractions_NoNorm(&data[0])
+
+    def set_unnormalized_mole_fractions(self, X):
+        """
+        Set the mole fractions without normalizing to force sum(X) == 1.0.
+        Useful primarily when calculating derivatives with respect to X[k]
+        by finite difference.
+        """
+        cdef np.ndarray[np.double_t, ndim=1] data
+        if len(X) == self.n_species:
+            data = np.ascontiguousarray(X, dtype=np.double)
+        else:
+            raise ValueError("Array has incorrect length")
+        self.thermo.setMoleFractions_NoNorm(&data[0])
+
     ######## Read-only thermodynamic properties ########
 
     property P: