diff --git a/rmgpy/molecule/molecule.py b/rmgpy/molecule/molecule.py
index f8193b5afa..fc3bd402a4 100644
--- a/rmgpy/molecule/molecule.py
+++ b/rmgpy/molecule/molecule.py
@@ -1164,7 +1164,7 @@ def number_of_surface_sites(self):
         cython.declare(count=cython.int)
         count = 0
         for atom in self.atoms:
-            if atom.symbol == 'X':
+            if atom.is_surface_site():
                 count += 1
         return count