--- a/python/mozbuild/mozbuild/backend/visualstudio.py
+++ b/python/mozbuild/mozbuild/backend/visualstudio.py
@@ -69,18 +69,20 @@ class VisualStudioBackend(CommonBackend)
     """
 
     def _init(self):
         CommonBackend._init(self)
 
         # These should eventually evolve into parameters.
         self._out_dir = os.path.join(self.environment.topobjdir, 'msvc')
         self._projsubdir = 'projects'
-        # But making this one a parameter requires testing first.
-        self._version = '2013'
+
+        self._version = self.environment.substs.get('MSVS_VERSION', None)
+        if not self._version:
+            raise Exception('MSVS_VERSION not defined; this should never happen')
 
         self._paths_to_sources = {}
         self._paths_to_includes = {}
         self._paths_to_defines = {}
         self._paths_to_configs = {}
         self._libs_to_paths = {}
         self._progs_to_paths = {}