diff --git a/examples/python/pincell/build-xml.py b/examples/python/pincell/build-xml.py
index 979614d635..2a550b9d73 100644
--- a/examples/python/pincell/build-xml.py
+++ b/examples/python/pincell/build-xml.py
@@ -51,7 +51,7 @@ o = openmc.Element('O')
 # Instantiate some Materials and register the appropriate Nuclides
 uo2 = openmc.Material(material_id=1, name='UO2 fuel at 2.4% wt enrichment')
 uo2.set_density('g/cm3', 10.29769)
-uo2.add_element(u, 1., enrichment=0.024)
+uo2.add_element(u, 1., enrichment=2.4)
 uo2.add_element(o, 2.)
 
 helium = openmc.Material(material_id=2, name='Helium for gap')
diff --git a/openmc/data/data.py b/openmc/data/data.py
index 1f759d3d4d..a3e7158126 100644
--- a/openmc/data/data.py
+++ b/openmc/data/data.py
@@ -1,5 +1,6 @@
 import itertools
 import os
+import re
 
 
 # Isotopic abundances from M. Berglund and M. E. Wieser, "Isotopic compositions
@@ -149,6 +150,7 @@ def atomic_mass(isotope):
 
     """
     if not _ATOMIC_MASS:
+
         # Load data from AME2012 file
         mass_file = os.path.join(os.path.dirname(__file__), 'mass.mas12')
         with open(mass_file, 'r') as ame:
@@ -159,6 +161,18 @@ def atomic_mass(isotope):
                     line[100:106] + '.' + line[107:112])
                 _ATOMIC_MASS[name.lower()] = mass
 
+        # For isotopes found in some libraries that represent all natural
+        # isotopes of their element (e.g. C0), calculate the atomic mass as
+        # the sum of the atomic mass times the natural abudance of the isotopes
+        # that make up the element.
+        for element in ['C', 'Zn', 'Pt', 'Os', 'Tl']:
+            isotope_zero = element.lower() + '0'
+            _ATOMIC_MASS[isotope_zero] = 0.
+            for iso, abundance in NATURAL_ABUNDANCE.items():
+                if re.match(r'{}\d+'.format(element), iso):
+                    _ATOMIC_MASS[isotope_zero] += abundance * \
+                                                  _ATOMIC_MASS[iso.lower()]
+
     # Get rid of metastable information
     if '_' in isotope:
         isotope = isotope[:isotope.find('_')]
diff --git a/openmc/material.py b/openmc/material.py
index c265c90694..70a070ddd6 100644
--- a/openmc/material.py
+++ b/openmc/material.py
@@ -64,6 +64,10 @@ class Material(object):
         List in which each item is a 3-tuple consisting of an
         :class:`openmc.Nuclide` instance, the percent density, and the percent
         type ('ao' or 'wo').
+    average_molar_mass : float
+        The average molar mass of nuclides in the material in units of grams per
+        mol.  For example, UO2 with 3 nuclides will have an average molar mass
+        of 270 / 3 = 90 g / mol.
 
     """
 
@@ -194,6 +198,27 @@ class Material(object):
     def distrib_otf_file(self):
         return self._distrib_otf_file
 
+    @property
+    def average_molar_mass(self):
+
+        # Get a list of all the nuclides, with elements expanded
+        nuclide_densities = self.get_nuclide_densities()
+
+        # Using the sum of specified atomic or weight amounts as a basis, sum
+        # the mass and moles of the material
+        mass = 0.
+        moles = 0.
+        for nuc, vals in nuclide_densities.items():
+            if vals[2] == 'ao':
+                mass += vals[1] * openmc.data.atomic_mass(nuc)
+                moles += vals[1]
+            else:
+                moles += vals[1] / openmc.data.atomic_mass(nuc)
+                mass += vals[1]
+
+        # Compute and return the molar mass
+        return mass / moles
+
     @id.setter
     def id(self, material_id):