diff --git a/actionpack/lib/action_view/helpers/prototype_helper.rb b/actionpack/lib/action_view/helpers/prototype_helper.rb
index 1a0e660d52..04bf5f2a30 100644
--- a/actionpack/lib/action_view/helpers/prototype_helper.rb
+++ b/actionpack/lib/action_view/helpers/prototype_helper.rb
@@ -1068,7 +1068,7 @@ module ActionView
     
       def build_observer(klass, name, options = {})
         if options[:with] && (options[:with] !~ /[\{=(.]/)
-          options[:with] = "'#{options[:with]}=' + value"
+          options[:with] = "'#{options[:with]}=' + encodeURIComponent(value)"
         else
           options[:with] ||= 'value' unless options[:function]
         end
diff --git a/actionpack/test/template/prototype_helper_test.rb b/actionpack/test/template/prototype_helper_test.rb
index 9a1079b297..5e00eadb8d 100644
--- a/actionpack/test/template/prototype_helper_test.rb
+++ b/actionpack/test/template/prototype_helper_test.rb
@@ -219,9 +219,9 @@ class PrototypeHelperTest < PrototypeHelperBaseTest
   end
   
   def test_observe_field_using_with_option
-    expected = %(<script type=\"text/javascript\">\n//<![CDATA[\nnew Form.Element.Observer('glass', 300, function(element, value) {new Ajax.Request('http://www.example.com/check_value', {asynchronous:true, evalScripts:true, parameters:'id=' + value})})\n//]]>\n</script>)
+    expected = %(<script type=\"text/javascript\">\n//<![CDATA[\nnew Form.Element.Observer('glass', 300, function(element, value) {new Ajax.Request('http://www.example.com/check_value', {asynchronous:true, evalScripts:true, parameters:'id=' + encodeURIComponent(value)})})\n//]]>\n</script>)
     assert_dom_equal expected, observe_field("glass", :frequency => 5.minutes, :url => { :action => "check_value" }, :with => 'id')
-    assert_dom_equal expected, observe_field("glass", :frequency => 5.minutes, :url => { :action => "check_value" }, :with => "'id=' + value")
+    assert_dom_equal expected, observe_field("glass", :frequency => 5.minutes, :url => { :action => "check_value" }, :with => "'id=' + encodeURIComponent(value)")
   end
   
   def test_observe_field_using_json_in_with_option