i like to use the ExpandoMetaClass to add properties and methods during runtime. For testing purposes I wrote: class Foo{ } println Foo.metaClass def a1 = new Foo(); println a1.metaClass.class Foo.metaClass.expando = true; println Foo.metaClass.class def a2 = new Foo(); println a2.metaClass.class a2.metaClass.test = "value" println a2.test println a2.metaClass.properties.name Unfortunately "println a2.metaClass.properties.name" returns ["expando", "test", "class", "expando", "metaClass", "test"] I think this is related to the getProperties method in ExpandoMetaClass: public List getProperties() { List propertyList = new ArrayList(); propertyList.addAll(expandoProperties.values()); propertyList.addAll(super.getProperties()); return propertyList; } At first it reads the properties form the local list and adds the properties for MetaClassImpl afterwards. Why are all expando properties are hold twice?