diff --git a/CHANGELOG b/CHANGELOG
index 2b0dbcc..dcf3fca 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,5 @@
+* Use different tactic to sniff for Opera in order to avoid false positives in IE. [Tobie Langel, jddalton]
+
 * Rename variable in Form.Element.Serializers.select. (jddalton)
 
 * Coerce Opera's version string into a number whenever we need to sniff. (Sam Holman, jddalton)
diff --git a/src/dom.js b/src/dom.js
index 07ef2d6..89e17ee 100644
--- a/src/dom.js
+++ b/src/dom.js
@@ -457,7 +457,7 @@ Element.Methods = {
       element.style.position = 'relative';
       // Opera returns the offset relative to the positioning context, when an
       // element is position relative but top and left have not been defined
-      if (window.opera) {
+      if (Prototype.Browser.Opera) {
         element.style.top = 0;
         element.style.left = 0;
       }  
diff --git a/src/prototype.js b/src/prototype.js
index d0a874c..2b0655e 100644
--- a/src/prototype.js
+++ b/src/prototype.js
@@ -4,10 +4,12 @@ var Prototype = {
   Version: '<%= PROTOTYPE_VERSION %>',
   
   Browser: {
-    IE:     !!(window.attachEvent && !window.opera),
-    Opera:  !!window.opera,
+    IE:     !!(window.attachEvent &&
+      navigator.userAgent.indexOf('Opera') === -1),
+    Opera:  navigator.userAgent.indexOf('Opera') > -1,
     WebKit: navigator.userAgent.indexOf('AppleWebKit/') > -1,
-    Gecko:  navigator.userAgent.indexOf('Gecko') > -1 && navigator.userAgent.indexOf('KHTML') == -1,
+    Gecko:  navigator.userAgent.indexOf('Gecko') > -1 && 
+      navigator.userAgent.indexOf('KHTML') === -1,
     MobileSafari: !!navigator.userAgent.match(/Apple.*Mobile.*Safari/)
   },