A system for enterprise information integration is described. The system is based on different layers: the data sources, source ontologies corresponding to the data schema of the data sources, an integration ontology integrating the different source ontologies, a level of query input, preferably in the form of a web service. Between the levels, mapping rules are used. For the mapping of document style web services and XML documents to ontologies detailed rules are given, covering i.a. simpleContent and complexContent elements, which are mapped to class properties and relations (object properties), respectively.