XML has become common in Internet-based application domains such as business-to-business and business-to-consumer applications. It has also formed a basis for service-oriented architectures such as Web services and the Semantic Web, particularly because ontology data employed in the Semantic Web are stored in XML. Measuring system coupling is a commonly accepted software engineering practice associated with producing high-quality software products. In many application domains, we can assess coupling in ontology-based systems before system development by measuring coupling in ontology data. (Because ontologies are built independently of applications that access them, they can be assessed for coupling before system development.) A proposed set of metrics measures coupling of ontology data in ontology-based systems represented in the Web Ontology Language (OWL), a derivative of XML. A real-world study demonstrates the metrics? use in integrating ontologies.