Abstract. UML class diagrams can be used as a language for expressing a conceptual model of a domain. We use the General Ontological Language (GOL) and its underlying upper level ontology, proposed in [1], to evaluate the ontological correctness of a conceptual UML class model and to develop guidelines for how the constructs of the UML should be used in conceptual modeling. In particular, we discuss the UML metaconcepts of classes and objects, powertypes, association and aggregation/composition from an ontological point of