diff --git a/CHANGELOG b/CHANGELOG
index 74f4cc4..0ca90ad 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,5 @@
+* For consistency: use Object.isUndefined where possible. [Tobie Langel]
+
 * Make String#isJSON return false for empty or blank strings. Make Ajax.Response#responseJSON null when Ajax.Response#responseText is empty or blank. [Andrew Dupont, Thomas Fuchs, Tobie Langel]
 
 * Make Ajax.Response#_getResponseJSON use Ajax.Response#responseText. [Tobie Langel]
diff --git a/src/ajax.js b/src/ajax.js
index c18c529..d7790b0 100644
--- a/src/ajax.js
+++ b/src/ajax.js
@@ -242,7 +242,7 @@ Ajax.Response = Class.create({
     
     if(readyState == 4) {
       var xml = transport.responseXML;
-      this.responseXML  = xml === undefined ? null : xml;
+      this.responseXML  = Object.isUndefined(xml) ? null : xml;
       this.responseJSON = this._getResponseJSON();
     }
   },
diff --git a/src/array.js b/src/array.js
index 41fc895..245ad13 100644
--- a/src/array.js
+++ b/src/array.js
@@ -100,7 +100,7 @@ Object.extend(Array.prototype, {
     var results = [];
     this.each(function(object) {
       var value = Object.toJSON(object);
-      if (value !== undefined) results.push(value);
+      if (!Object.isUndefined(value)) results.push(value);
     });
     return '[' + results.join(', ') + ']';
   }
diff --git a/src/base.js b/src/base.js
index cda0435..6629914 100644
--- a/src/base.js
+++ b/src/base.js
@@ -69,7 +69,7 @@ Object.extend = function(destination, source) {
 Object.extend(Object, {
   inspect: function(object) {
     try {
-      if (object === undefined) return 'undefined';
+      if (Object.isUndefined(object)) return 'undefined';
       if (object === null) return 'null';
       return object.inspect ? object.inspect() : object.toString();
     } catch (e) {
@@ -94,7 +94,7 @@ Object.extend(Object, {
     var results = [];
     for (var property in object) {
       var value = Object.toJSON(object[property]);
-      if (value !== undefined)
+      if (!Object.isUndefined(value))
         results.push(property.toJSON() + ': ' + value);
     }
     
@@ -163,7 +163,7 @@ Object.extend(Function.prototype, {
   },
   
   bind: function() {
-    if (arguments.length < 2 && arguments[0] === undefined) return this;
+    if (arguments.length < 2 && Object.isUndefined(arguments[0])) return this;
     var __method = this, args = $A(arguments), object = args.shift();
     return function() {
       return __method.apply(object, args.concat($A(arguments)));
diff --git a/src/dom.js b/src/dom.js
index b480015..49cc145 100644
--- a/src/dom.js
+++ b/src/dom.js
@@ -286,7 +286,7 @@ Element.Methods = {
     var attributes = { }, t = Element._attributeTranslations.write;
     
     if (typeof name == 'object') attributes = name;
-    else attributes[name] = value === undefined ? true : value;
+    else attributes[name] = Object.isUndefined(value) ? true : value;
     
     for (var attr in attributes) {
       var name = t.names[attr] || attr, value = attributes[attr];
@@ -412,7 +412,7 @@ Element.Methods = {
       if (property == 'opacity') element.setOpacity(styles[property]);
       else 
         elementStyle[(property == 'float' || property == 'cssFloat') ?
-          (elementStyle.styleFloat === undefined ? 'cssFloat' : 'styleFloat') : 
+          (Object.isUndefined(elementStyle.styleFloat) ? 'cssFloat' : 'styleFloat') : 
             property] = styles[property];
 
     return element;
diff --git a/src/enumerable.js b/src/enumerable.js
index 50d09d2..45ace8e 100644
--- a/src/enumerable.js
+++ b/src/enumerable.js
@@ -102,7 +102,7 @@ var Enumerable = {
   },
   
   inGroupsOf: function(number, fillWith) {
-    fillWith = fillWith === undefined ? null : fillWith;
+    fillWith = Object.isUndefined(fillWith) ? null : fillWith;
     return this.eachSlice(number, function(slice) {
       while(slice.length < number) slice.push(fillWith);
       return slice;
@@ -129,7 +129,7 @@ var Enumerable = {
     var result;
     this.each(function(value, index) {
       value = iterator(value, index);
-      if (result == undefined || value >= result)
+      if (result == null || value >= result)
         result = value;
     });
     return result;
@@ -140,7 +140,7 @@ var Enumerable = {
     var result;
     this.each(function(value, index) {
       value = iterator(value, index);
-      if (result == undefined || value < result)
+      if (result == null || value < result)
         result = value;
     });
     return result;
diff --git a/src/form.js b/src/form.js
index 0d823e8..0fd2ef3 100644
--- a/src/form.js
+++ b/src/form.js
@@ -6,7 +6,7 @@ var Form = {
   
   serializeElements: function(elements, options) {
     if (typeof options != 'object') options = { hash: !!options };
-    else if (options.hash === undefined) options.hash = true;
+    else if (Object.isUndefined(options.hash)) options.hash = true;
     var key, value, submitted = false, submit = options.submit;
     
     var data = elements.inject({ }, function(result, element) {
@@ -204,17 +204,17 @@ Form.Element.Serializers = {
   },
 
   inputSelector: function(element, value) {
-    if (value === undefined) return element.checked ? element.value : null;
+    if (Object.isUndefined(value)) return element.checked ? element.value : null;
     else element.checked = !!value;
   },
 
   textarea: function(element, value) {
-    if (value === undefined) return element.value;
+    if (Object.isUndefined(value)) return element.value;
     else element.value = value;
   },
   
   select: function(element, index) {
-    if (index === undefined)
+    if (Object.isUndefined(index))
       return this[element.type == 'select-one' ? 
         'selectOne' : 'selectMany'](element);
     else {
diff --git a/src/string.js b/src/string.js
index 58ef999..30d0ffb 100644
--- a/src/string.js
+++ b/src/string.js
@@ -31,7 +31,7 @@ Object.extend(String.prototype, {
   
   sub: function(pattern, replacement, count) {
     replacement = this.gsub.prepareReplacement(replacement);
-    count = count === undefined ? 1 : count;
+    count = Object.isUndefined(count) ? 1 : count;
     
     return this.gsub(pattern, function(match) {
       if (--count < 0) return match[0];
@@ -46,7 +46,7 @@ Object.extend(String.prototype, {
   
   truncate: function(length, truncation) {
     length = length || 30;
-    truncation = truncation === undefined ? '...' : truncation;
+    truncation = Object.isUndefined(truncation) ? '...' : truncation;
     return this.length > length ? 
       this.slice(0, length - truncation.length) + truncation : String(this);
   },