Altought the release 1.1.1 added substitutionGroups support, the Castor version 1.1.2.1 doesn't seem to do anything for substitution groups: <xsd:element name="A" type="cnv:A_T" /> <xsd:element name="B" substitutionGroup="cnv:A" type="cnv:A_T" /> <xsd:complexType name="A_T" /> <xsd:complexType name="MyClass"> <xsd:sequence> <xsd:element ref="cnv:A" /> </xsd:sequence> </xsd:complexType> the resulted java code is: A extending A_T, B extending A_T and MyClass has the property of type A. Obviously, unmarhalling <MyClass> <B /> </MyClass> fails.