The Unified Enterprise Modelling Language (UEML) aims to become a hub for integrated use of enterprise and information systems (IS) models expressed using different languages. A central part of this hub is an extendible ontology into which modelling languages and their constructs can be mapped, so that precise semantic relations between the languages and constructs can be established by comparing their ontology mappings. The paper presents and discusses ongoing work on reformulating the UEML ontology as an OWL2 DL ontology, the Unified Enterprise Modelling Ontology (UEMO).