diff --git a/lib/mongo/util/bson.rb b/lib/mongo/util/bson.rb
index 53af04b..6569521 100644
--- a/lib/mongo/util/bson.rb
+++ b/lib/mongo/util/bson.rb
@@ -80,9 +80,9 @@ class BSON
     @buf.put_int(0)
 
     # Write key/value pairs. Always write _id first if it exists.
-    oid = obj.delete('_id') || obj.delete(:_id)
+    oid = obj['_id'] || obj[:_id]
     serialize_key_value('_id', oid) if oid
-    obj.each {|k, v| serialize_key_value(k, v) }
+    obj.each {|k, v| serialize_key_value(k, v) unless k == '_id' || k == :_id }
 
     serialize_eoo_element(@buf)
     @buf.put_int(@buf.size, 0)
diff --git a/mongo-ruby-driver.gemspec b/mongo-ruby-driver.gemspec
index e66d07d..3ee6624 100644
--- a/mongo-ruby-driver.gemspec
+++ b/mongo-ruby-driver.gemspec
@@ -1,6 +1,6 @@
 Gem::Specification.new do |s|
   s.name = 'mongo'
-  s.version = '0.3.0'
+  s.version = '0.3.1'
   s.platform = Gem::Platform::RUBY
   s.summary = 'Simple pure-Ruby driver for the 10gen Mongo DB'
   s.description = 'A pure-Ruby driver for the 10gen Mongo DB. For more information about Mongo, see http://www.mongodb.org.'
diff --git a/tests/test_bson.rb b/tests/test_bson.rb
index f214c38..e58b035 100644
--- a/tests/test_bson.rb
+++ b/tests/test_bson.rb
@@ -155,4 +155,18 @@ class BSONTest < Test::Unit::TestCase
     assert_equal '_id', roundtrip.keys.first
   end
 
+  def test_do_not_change_original_object
+    val = OrderedHash.new
+    val['not_id'] = 1
+    val['_id'] = 2
+    assert val.keys.include?('_id')
+    @b.serialize(val)
+    assert val.keys.include?('_id')
+
+    val = {'a' => 'foo', 'b' => 'bar', :_id => 42, 'z' => 'hello'}
+    assert val.keys.include?(:_id)
+    @b.serialize(val)
+    assert val.keys.include?(:_id)
+  end
+
 end