The increasing utilization of business process models both in business analysis and information systems development raises several issues regarding quality measures. In this context, this paper discusses understandability as a particular quality aspect and its connection with personal, model, and content related factors. We use an online survey to explore the ability of the model reader to draw correct conclusions from a set of process models. For the first group of the participants we els with abstract activity labels (e.g. A, B, C) while the second group received the same models with illustrative labels such as "check credit limit". The results suggest that all three categories indeed have an impact on the understandability.