diff --git a/README.md b/README.md
index b1a613f..99a5cc8 100644
--- a/README.md
+++ b/README.md
@@ -42,6 +42,7 @@ encoded private key for RSA and ECDSA. In case of a private key with passphrase
 * `noTimestamp`
 * `header`
 * `keyid`
+* `mutatePayload`: if true, the sign function will modify the payload object directly. This is useful if you need a raw reference to the payload after claims have been applied to it but before it has been encoded into a token.
 
 If `payload` is not a buffer or a string, it will be coerced into a string using `JSON.stringify`.
 
diff --git a/sign.js b/sign.js
index d71189b..ea3bb36 100644
--- a/sign.js
+++ b/sign.js
@@ -20,7 +20,8 @@ var sign_options_schema = {
   subject: { isValid: isString, message: '"subject" must be a string' },
   jwtid: { isValid: isString, message: '"jwtid" must be a string' },
   noTimestamp: { isValid: isBoolean, message: '"noTimestamp" must be a boolean' },
-  keyid: { isValid: isString, message: '"keyid" must be a string' }
+  keyid: { isValid: isString, message: '"keyid" must be a string' },
+  mutatePayload: { isValid: isBoolean, message: '"mutatePayload" must be a boolean' }
 };
 
 var registered_claims_schema = {
@@ -110,7 +111,9 @@ module.exports = function (payload, secretOrPrivateKey, options, callback) {
     catch (error) {
       return failure(error);
     }
-    payload = xtend(payload);
+    if (!options.mutatePayload) {
+      payload = xtend(payload);
+    }
   } else {
     var invalid_options = options_for_objects.filter(function (opt) {
       return typeof options[opt] !== 'undefined';
diff --git a/test/async_sign.tests.js b/test/async_sign.tests.js
index b6cc052..d22f974 100644
--- a/test/async_sign.tests.js
+++ b/test/async_sign.tests.js
@@ -82,6 +82,30 @@ describe('signing a token asynchronously', function() {
       });
     });
 
+    describe('when mutatePayload is not set', function() {
+      it('should not apply claims to the original payload object (mutatePayload defaults to false)', function(done) {
+        var originalPayload = { foo: 'bar' };
+        jwt.sign(originalPayload, 'secret', { notBefore: 60, expiresIn: 600 }, function (err) {
+          if (err) { return done(err); }
+          expect(originalPayload).to.not.have.property('nbf');
+          expect(originalPayload).to.not.have.property('exp');
+          done();
+        });
+      });
+    });
+
+    describe('when mutatePayload is set to true', function() {
+      it('should apply claims directly to the original payload object', function(done) {
+        var originalPayload = { foo: 'bar' };
+        jwt.sign(originalPayload, 'secret', { notBefore: 60, expiresIn: 600, mutatePayload: true }, function (err) {
+          if (err) { return done(err); }
+          expect(originalPayload).to.have.property('nbf').that.is.a('number');
+          expect(originalPayload).to.have.property('exp').that.is.a('number');
+          done();
+        });
+      });
+    });
+
     describe('secret must have a value', function(){
       [undefined, '', 0].forEach(function(secret){
         it('should return an error if the secret is falsy and algorithm is not set to none: ' + (typeof secret === 'string' ? '(empty string)' : secret), function(done) {