Index: pym/portage_util.py
===================================================================
--- pym/portage_util.py	(revision 10292)
+++ pym/portage_util.py	(working copy)
@@ -306,7 +306,13 @@
 			return (newfile, StringIO.StringIO())
 
 def getconfig(mycfg, tolerant=0, allow_sourcing=False, expand=True):
-	mykeys={}
+	if isinstance(expand, dict):
+		# Some existing variable definitions have been
+		# passed in, for use in substitutions.
+		mykeys = expand.copy()
+		expand = True
+	else:
+		mykeys = {}
 	try:
 		f=open(mycfg,'r')
 	except IOError, e:
Index: pym/portage.py
===================================================================
--- pym/portage.py	(revision 10292)
+++ pym/portage.py	(working copy)
@@ -1440,8 +1440,13 @@
 			self.make_defaults_use = []
 			self.mygcfg = {}
 			if self.profiles:
-				mygcfg_dlists = [getconfig(os.path.join(x, "make.defaults")) \
-					for x in self.profiles]
+				mygcfg_dlists = []
+				var_map = {}
+				for x in self.profiles:
+					var_map = getconfig(os.path.join(x, "make.defaults"),
+						expand=var_map)
+					mygcfg_dlists.append(var_map)
+
 				for cfg in mygcfg_dlists:
 					if cfg:
 						self.make_defaults_use.append(cfg.get("USE", ""))