diff --git a/activemodel/CHANGELOG.md b/activemodel/CHANGELOG.md
index 5135e47db1af0a92f1caa9be4eb35eb677b6c701..dc39917984cc52b1a91e31a360896433391f9200 100644
--- a/activemodel/CHANGELOG.md
+++ b/activemodel/CHANGELOG.md
@@ -1,5 +1,7 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   Specify type of singular association during serialization *Steve Klabnik*
+
 *   Fixed length validator to correctly handle nil values. Fixes #7180.
 
     *Michal Zima*
diff --git a/activemodel/lib/active_model/serializers/xml.rb b/activemodel/lib/active_model/serializers/xml.rb
index fb6093cce5a4e835a1f0a6dd529166840c9574a1..4a17a63e208aa71ad355e3ea5cf016b30162a0a1 100755
--- a/activemodel/lib/active_model/serializers/xml.rb
+++ b/activemodel/lib/active_model/serializers/xml.rb
@@ -149,7 +149,12 @@ def add_associations(association, records, opts)
             end
           else
             merged_options[:root] = association.to_s
-            records.to_xml(merged_options)
+
+            unless records.class.to_s.underscore == association.to_s
+              merged_options[:type] = records.class.name
+            end
+
+            records.to_xml merged_options
           end
         end
 
diff --git a/activemodel/test/cases/serializers/xml_serialization_test.rb b/activemodel/test/cases/serializers/xml_serialization_test.rb
index 90ddf8ff0cc5933a6e389a06531b8d5f16474347..99a9c1fe33a3c44687c7ebf833d0c84d8786278c 100755
--- a/activemodel/test/cases/serializers/xml_serialization_test.rb
+++ b/activemodel/test/cases/serializers/xml_serialization_test.rb
@@ -6,12 +6,12 @@
 class Contact
   include ActiveModel::Serializers::Xml
 
-  attr_accessor :address, :friends
+  attr_accessor :address, :friends, :contact
 
   remove_method :attributes if method_defined?(:attributes)
 
   def attributes
-    instance_values.except("address", "friends")
+    instance_values.except("address", "friends", "contact")
   end
 end
 
@@ -56,6 +56,9 @@ def setup
     @contact.address.zip = 11111
     @contact.address.apt_number = 35
     @contact.friends = [Contact.new, Contact.new]
+    @related_contact = SerializableContact.new
+    @related_contact.name = "related"
+    @contact.contact = @related_contact
   end
 
   test "should serialize default root" do
@@ -256,4 +259,9 @@ def to_ary
     assert_match %r{<address>}, xml
     assert_match %r{<apt-number type="integer">}, xml
   end
+
+  test "association with sti" do
+    xml = @contact.to_xml(include: :contact)
+    assert xml.include?(%(<contact type="SerializableContact">))
+  end
 end