diff --git a/openmc/element.py b/openmc/element.py
index c391d08235..14c98d4952 100644
--- a/openmc/element.py
+++ b/openmc/element.py
@@ -126,7 +126,7 @@ class Element(object):
         """
 
         isotopes = []
-        for isotope, abundance in natural_abundance.items():
+        for isotope, abundance in sorted(natural_abundance.items()):
             if isotope.startswith(self.name + '-'):
                 nuc = openmc.Nuclide(isotope, self.xs)
                 isotopes.append((nuc, abundance))