diff --git a/lib/sourcemaps.js b/lib/sourcemaps.js
index f31e279..9b81d9a 100644
--- a/lib/sourcemaps.js
+++ b/lib/sourcemaps.js
@@ -45,7 +45,7 @@ exports.concatenateSourceMaps = function(outFile, mapsWithOffsets, basePath, sou
 
     if (sourceMapContents && map.sourcesContent) {
       for (var i=0; i<map.sources.length; i++) {
-        var source = (map.sourceRoot || '') + map.sources[i];
+        var source = path.normalize(path.isAbsolute(map.sources[i]) ? map.sources[i] : (map.sourceRoot || '') + map.sources[i]).replace(/\\/g, '/');
         if (!source.match(/\/@traceur/)) {
           if (!contentsBySource[source]) {
             contentsBySource[source] = map.sourcesContent[i];
@@ -98,7 +98,7 @@ exports.concatenateSourceMaps = function(outFile, mapsWithOffsets, basePath, sou
     if (isFileURL(source))
       source = fromFileURL(source);
 
-    return path.relative(outPath, path.resolve(basePath, source)).replace(/\\/g, '/');
+    return path.isAbsolute(source) ? source : path.relative(outPath, path.resolve(basePath, source)).replace(/\\/g, '/');
   });
 
   return JSON.stringify(normalized);