The problem that XML formally governs syntax only but not semantics has been recognized as a serious barrier for XML-Based data integration and the extension of current Web to the semantic Web. To address this problem, the XML Semantics Definition Language (XSDL) is proposed to explicitly express the XML author’s intended meaning and a model-theoretic semantics for XML. In this way, the XML becomes a sub-language of RDF (resource description framework) in expressivity and the XML data can be semantics-preserving transformed to the RDF data. The semantic validity and entailment problem of XML documents are further provided and they are reduced to the knowledge base unsatisfiability problem in description logic language ΣΗΟΙΝ(?).