Recently, the interest in the use of ontologies — which can be seen as formal representations of conceptual models — has increased because of the excitement about the vision of a “Semantic Web”. When ontologies are used on the web, the distributed and dynamic nature of it requires advanced support for change management. This paper discusses the working of OntoView, a web-based change management system for ontologies. OntoView provides a transparent interface to different versions of ontologies, by maintaining not only the transformations between them, but also the conceptual relation between concepts in different versions. It uses several rules to find changes in ontologies and it visualizes them — and some of their possible consequences — in the file representations. The user is able to specify the conceptual implication of the differences, which allows the interoperability of data that is described by the ontologies. This paper briefly describes the system and presents the mechanism that we used to find and classify changes in RDFS / DAML ontologies. It also shows how users can specify the conceptual implication of changes to help interoperability.