Knowledge is of general utility and should be captured thinking in reuse. A key idea underlining knowledge capturing for reuse is to consider that there are two major kinds of knowledge: domain and task knowledge. Ontologies can be used for representing both kinds of knowledge. However, while domain ontologies are broadly used and there are many proposals of models for representing them, the same does not occur for task ontologies. In this paper we propose the use of UML activity diagrams for capturing task control-flow, and UML class diagrams for capturing the knowledge roles involved in its activities. We also discuss the interrelationship between these two models and how they can be combined with domain ontologies in order to describe the knowledge involved in a class of applications.