Abstract. Models have been playing a major role in information system engineering and interoperability: in this paper, we put an emphasis on the complementary role of metamodels. Metamodels are used as a core of two innovative platforms for information system interoperability. The first platform allows to combine formal and informal approaches for both modeling and interoperability of information systems. The second platform uses metamodeling and reengineering in order to reuse interoperability know-how in the Semantic Web. Both platforms are based on metamodels that are integrated from metamodels of participating information systems. Such integration of metamodels is carried out by using a UML-based metamodeling architecture we have proposed.