diff --git a/README.md b/README.md
index 50009e4..5e639ea 100644
--- a/README.md
+++ b/README.md
@@ -12,7 +12,7 @@ Install the module with: `npm install base64-arraybuffer`
 ## API
 The library encodes and decodes base64 to and from ArrayBuffers
 
- - __encode(buffer)__ - Encodes `ArrayBuffer` into base64 string
+ - __encode(buffer [, start, [length]])__ - Encodes `ArrayBuffer` into base64 string
  - __decode(str)__ - Decodes base64 string to `ArrayBuffer`
 
 ## License
diff --git a/lib/base64-arraybuffer.js b/lib/base64-arraybuffer.js
index 362fbfa..ff5bd03 100644
--- a/lib/base64-arraybuffer.js
+++ b/lib/base64-arraybuffer.js
@@ -8,8 +8,13 @@
 (function(chars){
   "use strict";
 
-  exports.encode = function(arraybuffer) {
-    var bytes = new Uint8Array(arraybuffer),
+  exports.encode = function(arraybuffer, start, length) {
+
+    // use != undefined to include both 'undefined' and 'null'
+    start = (start != undefined) ? start : 0;
+    length = (length != undefined) ? length : arraybuffer.byteLength - start;
+
+    var bytes = new Uint8Array(arraybuffer, start, length),
     i, len = bytes.length, base64 = "";
 
     for (i = 0; i < len; i+=3) {