The domination of XML in the Internet for data exchange has led to the development of standards with XML Schema syntax for several application domains. Advanced semantic support, provided by domain ontolo gies and semantic Web tools like logic-based reasoners, is still very useful for many applications. In order to provide it, interoperability between XML Schema and OWL is necessary so that XML schemas can be converted to OWL. This way, the semantics of the standards can be enriched with domain knowledge encoded in OWL domain ontologies and further semantic process ing may take place. In order to achieve interoperability between XML Schema and OWL, we have developed XS2OWL, a model and a system that are presented in this paper and enable the automatic transformation of XML Schemas in OWL-DL. XS2OWL also enables the consistent transformation of the derived knowledge (individuals) from OWL-DL to XML constructs that obey the original XML Schemas.