diff --git a/interfaces/cython/cantera/_cantera.pxd b/interfaces/cython/cantera/_cantera.pxd
index 92bbcac15..07cc19d0f 100644
--- a/interfaces/cython/cantera/_cantera.pxd
+++ b/interfaces/cython/cantera/_cantera.pxd
@@ -229,6 +229,7 @@ cdef extern from "cantera/thermo/SurfPhase.h":
         void setSiteDensity(double) except +
         void setCoverages(double*) except +
         void setCoveragesByName(Composition&) except +
+        void setCoveragesNoNorm(double*) except +
         void getCoverages(double*) except +
 
 
diff --git a/interfaces/cython/cantera/thermo.pyx b/interfaces/cython/cantera/thermo.pyx
index 2ee21d178..3f3e6c265 100644
--- a/interfaces/cython/cantera/thermo.pyx
+++ b/interfaces/cython/cantera/thermo.pyx
@@ -1191,6 +1191,20 @@ cdef class InterfacePhase(ThermoPhase):
                 np.ascontiguousarray(theta, dtype=np.double)
             self.surf.setCoverages(&data[0])
 
+    def set_unnormalized_coverages(self, cov):
+        """
+        Set the surface coverages without normalizing to force sum(cov) == 1.0.
+        Useful primarily when calculating derivatives with respect to cov[k] by
+        finite difference.
+        """
+        cdef np.ndarray[np.double_t, ndim=1] data
+        if len(cov) == self.n_species:
+            data = np.ascontiguousarray(cov, dtype=np.double)
+        else:
+            raise ValueError("Array has incorrect length."
+                 " Got {}, expected {}.".format(len(cov), self.n_species))
+        self.surf.setCoveragesNoNorm(&data[0])
+
 
 cdef class PureFluid(ThermoPhase):
     """