diff --git a/README.md b/README.md
index b8868ad923..f175700d17 100644
--- a/README.md
+++ b/README.md
@@ -148,10 +148,16 @@ to5.transformFile("filename.js", options, function (err, result) {
   // If truthy, adds a `map` property to returned output.
   // If set to "inline", a comment with a sourceMappingURL directive is added to
   // the bottom of the returned code.
-  sourceMap: false,
+  // Default: false
+  sourceMap: true,
+
+  // Adds an `ast` property to returned output containing the ast tree used.
+  // Default: false
+  ast: true,
 
   // Filename for use in errors etc.
-  filename: "unknown"
+  // Default: "unknown"
+  filename: "filename"
 }
 ```
 
diff --git a/lib/6to5/transform.js b/lib/6to5/transform.js
index f27d3d0fbb..3005eb6621 100644
--- a/lib/6to5/transform.js
+++ b/lib/6to5/transform.js
@@ -1,7 +1,8 @@
-var traverse = require("./traverse");
-var recast   = require("recast");
-var util     = require("./util");
-var _        = require("lodash");
+var sourceMap = require("source-map");
+var traverse  = require("./traverse");
+var recast    = require("recast");
+var util      = require("./util");
+var _         = require("lodash");
 
 var ensureTransformerNames = function (type, keys) {
   _.each(keys, function (key) {
@@ -20,7 +21,8 @@ var transform = module.exports = function (code, opts) {
     whitelist: [],
     sourceMap: false,
     filename:  "unknown",
-    format:    {}
+    format:    {},
+    ast:       false
   });
 
   ensureTransformerNames("blacklist", opts.blacklist);
@@ -56,6 +58,14 @@ transform._run = function (code, tree, opts) {
     result.code += "\n" + util.sourceMapToComment(result.map);
   }
 
+  result.map = result.map || null;
+
+  if (opts.ast) {
+    result.ast = tree;
+  } else {
+    result.ast = null;
+  }
+
   return result;
 };
 
@@ -76,25 +86,29 @@ transform.test = function (task, assert) {
   var opts   = task.options;
 
   opts.filename = actual.filename;
+  opts.ast      = true;
 
   var actualCode = actual.code.trim();
 
-  var transformedResult = transform(actualCode, opts);
-  var transformedCode = transformedResult.code;
+  var actualResult = transform(actualCode, opts);
+  var actualAst    = actualResult.ast;
+  actualCode       = recast.prettyPrint(actualAst).code;
 
-  var actualAst = util.parse(actual.filename, transformedCode);
-  var actualResult = recast.prettyPrint(actualAst);
-  actualCode = actualResult.code;
-
-  var expectCode = expect.code.trim();
-  var expectAst = util.parse(expect.filename, expectCode);
+  var expectCode   = expect.code.trim();
+  var expectAst    = util.parse(expect.filename, expectCode);
   var expectResult = recast.prettyPrint(expectAst);
-  expectCode = expectResult.code;
+  expectCode       = expectResult.code;
 
   assert.equal(actualCode, expectCode);
 
-  if (opts.sourceMap && task.sourceMap) {
-    assert.deepEqual(util.formatJSON(transformedResult.map), util.formatJSON(task.sourceMap));
+  if (task.sourceMappings) {
+    var consumer = new sourceMap.SourceMapConsumer(actualResult.map);
+
+    _.each(task.sourceMappings, function (mapping, i) {
+      var pos = consumer.originalPositionFor(mapping.generated);
+      var msg = "source mapping " + ++i + " - generated: " + mapping.generated.line + ":" + mapping.generated.column;
+      assert.equal(pos.line + ":" + pos.column, mapping.original.line + ":" + mapping.original.column, msg);
+    });
   }
 };
 
diff --git a/package.json b/package.json
index aa558be67a..3084236f01 100644
--- a/package.json
+++ b/package.json
@@ -58,6 +58,7 @@
     "jstransform": "^6.3.2",
     "uglify-js": "^2.4.15",
     "browserify": "^6.0.3",
-    "proclaim": "^2.0.0"
+    "proclaim": "^2.0.0",
+    "source-map": "^0.1.40"
   }
 }
diff --git a/test/_helper.js b/test/_helper.js
index 1fdecca05e..0777a87609 100644
--- a/test/_helper.js
+++ b/test/_helper.js
@@ -71,7 +71,6 @@ exports.getTests = function () {
       var taskOpts = _.merge({ filename: actualLoc }, _.cloneDeep(suite.options));
       if (fs.existsSync(taskOptsLoc)) _.merge(taskOpts, require(taskOptsLoc));
 
-
       var test = {
         title: humanise(taskName),
         options: taskOpts,
@@ -87,10 +86,10 @@ exports.getTests = function () {
 
       suite.tests.push(test);
 
-      var sourceMapLoc = taskDir + "/source-map.json";
-      if (fs.existsSync(sourceMapLoc)) {
+      var sourceMappingsLoc = taskDir + "/source-mappings.json";
+      if (fs.existsSync(sourceMappingsLoc)) {
         test.options.sourceMap = true;
-        test.sourceMap = require(sourceMapLoc);
+        test.sourceMappings = require(sourceMappingsLoc);
       }
     });
   });
diff --git a/test/fixtures/source-maps/simple/actual.js b/test/fixtures/source-maps/arrow-function/actual.js
similarity index 100%
rename from test/fixtures/source-maps/simple/actual.js
rename to test/fixtures/source-maps/arrow-function/actual.js
diff --git a/test/fixtures/source-maps/arrow-function/expected.js b/test/fixtures/source-maps/arrow-function/expected.js
new file mode 100644
index 0000000000..3ce826a33f
--- /dev/null
+++ b/test/fixtures/source-maps/arrow-function/expected.js
@@ -0,0 +1,3 @@
+var t = function(x) {
+  return x * x;
+};
diff --git a/test/fixtures/source-maps/arrow-function/source-mappings.json b/test/fixtures/source-maps/arrow-function/source-mappings.json
new file mode 100644
index 0000000000..88886e7423
--- /dev/null
+++ b/test/fixtures/source-maps/arrow-function/source-mappings.json
@@ -0,0 +1,10 @@
+[{
+  "original": {
+    "line": 1,
+    "column": 15
+  },
+  "generated": {
+    "line": 2,
+    "column": 11
+  }
+}]
diff --git a/test/fixtures/source-maps/class/actual.js b/test/fixtures/source-maps/class/actual.js
new file mode 100644
index 0000000000..a1a7204a7d
--- /dev/null
+++ b/test/fixtures/source-maps/class/actual.js
@@ -0,0 +1,8 @@
+class Test {
+  get bar() {
+    throw new Error("wow");
+  }
+}
+
+var test = new Test;
+test.bar;
diff --git a/test/fixtures/source-maps/class/expected.js b/test/fixtures/source-maps/class/expected.js
new file mode 100644
index 0000000000..012e6129cd
--- /dev/null
+++ b/test/fixtures/source-maps/class/expected.js
@@ -0,0 +1,16 @@
+var Test = function() {
+  function Test() {}
+
+  Object.defineProperties(Test.prototype, {
+    bar: {
+      get: function() {
+        throw new Error("wow");
+      }
+    }
+  });
+
+  return Test;
+}();
+
+var test = new Test;
+test.bar;
diff --git a/test/fixtures/source-maps/class/source-mappings.json b/test/fixtures/source-maps/class/source-mappings.json
new file mode 100644
index 0000000000..b3f7bdab14
--- /dev/null
+++ b/test/fixtures/source-maps/class/source-mappings.json
@@ -0,0 +1,10 @@
+[{
+  "original": {
+    "line": 3,
+    "column": 11
+  },
+  "generated": {
+    "line": 7,
+    "column": 15
+  }
+}]
diff --git a/test/fixtures/source-maps/simple/expected.js b/test/fixtures/source-maps/simple/expected.js
deleted file mode 100644
index f7f53950c8..0000000000
--- a/test/fixtures/source-maps/simple/expected.js
+++ /dev/null
@@ -1 +0,0 @@
-var t = function (x) { return x * x; };
diff --git a/test/fixtures/source-maps/simple/source-map.json b/test/fixtures/source-maps/simple/source-map.json
deleted file mode 100644
index 7246019e42..0000000000
--- a/test/fixtures/source-maps/simple/source-map.json
+++ /dev/null
@@ -1,12 +0,0 @@
-{
-  "version": 3,
-  "file": "actual.js",
-  "sources": [
-    "actual.js"
-  ],
-  "names": [],
-  "mappings": "AAAA,CAAC,CAAC,EAAE,EAAE,WAAE;SAAK,EAAE,EAAE;CAAC",
-  "sourcesContent": [
-    "var t = x => x * x;"
-  ]
-}