Abstract. Usability is an important criterion for measuring and comparing the quality of software systems. It is particularly important for interactive verification systems, which heavily rely on user support to find proofs and that require various complex user interactions. In this paper, we present a questionnaire for evaluating interactive verification systems based on Green and Petre’s Cognitive Dimensions. In a first case study, we have used this questionnaire to evaluate our own tool, the KeY System. The lessons learned from this evaluation relate (1) to the usability of the KeY System and interactive verification systems in general and also (2) gave us insights on how to perform usability evaluations for interactive verification systems.