In this paper we describe how XML documents are mapped into an OWL ontology and how SWRL rules are used to validate the semantic content of XML documents. XML completion and data constraints are specified with SWRL. The semantic completion of the XML document can be mapped into a semantic completion of the corresponding ontology. Besides, SWRL serves for specifying and reasoning with data constraints. We will illustrate our approach with an example that shows that user intervention is vital to XML mapping and completion and SWRL helps to detect relevant data constraints. The approach has been tested with the well-known Protégé tool.