Abstract. With the rise of global networks like the Internet the importance of workflow systems is growing. However, security questions in such environments often only address secure communication. Another important topic that is often ignored is the separation of duties to prevent fraud within an organization. This paper introduces a model for separation of duties in workflows that have been specified with Petri nets. Rules will be given as facts of a logic program and expressed in propositional logic. The program allows for simulating and analyzing workflows and their security rules during build time. Keywords. Logical programming, Petri net, separation of duties, workflow