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XML has evolved to the format of choice for exposing data over the Web. Together with mature and maturing standards for querying XML (XSLT, XPath, and XQuery) the basic infrastructure for integrating multiple heterogeneous data sources is there. However, the versatility of XML as a data model and the unrestricted expressive power of XML query languages can lead to rather complex integration architectures, where low level syntactic heterogeneities and semantic heterogeneities are overcome all at once by means of complex query expressions. This paper explores how the Web Ontology Language OWL can be used as a more abstract modelling layer on top of XML data sources, described by an XML Schema, to which extent the semantic relationships provided by OWL can be used for mapping heterogeneous data sources to a common global schema, and how the inference mechanisms of OWL can be used to check the consistency of such mappings. Moreover, it introduces a query language for OWL as a natural extension of XQuery, and describes how these queries against a global schema are translated to XQueries against the original data sources.