Use of XML holds great promise for standardizing data models for realizing benefits such as lowered development costs and time for integrating inter-organizational business processes and intra-organizational knowledge management. Further benefits can be realized by formally defining common semantics in ontologies using the standardized models. Automation of business processes that require sharing knowledge represented in XML-based ontologies can then be supported. In this paper, a proof-of-concept application for using ontologies to support deduction of knowledge implicit in existing XML documents is presented. This system, called XML-hoo!, employs a customized portal user interface to answer queries about Shakespearian plays. Queries are answered by applying inference rules about these plays represented as axioms that comprise a Shakespearian ontology, composed of terminology corresponding to existing XML DTD's. These rules are applied to plays represented in XML that are in the public domain. Hence, answers to queries such as, isWho is Romeo's father?lt can be automatically deduced even though facts required for such answers are not explicitly structured in XML documents. This application demonstrates use of re-usable and sharable ontology representations to further leverage the expected proliferation of XML documents.