Heuristically Increasing the Axiomatization Coverage of Program Verification Systems Bernhard Beckert, Thorsten Bormer, and Markus Wagner The correctness of program verification systems is of great importance, as they are used to formally prove that safety- and security-critical programs follow their specification. This correctness needs to be checked and demonstrated to users and certification agencies. One of the contributing factors to the correctness of the whole verification system is the correctness of the background axiomatization, which captures the semantics of the program language. We present an optimisation framework for the maximization of the proportion of the axiomatization that is used ("covered") during testing. As the creation of test cases is a very time consuming process, we show how to increase the coverage for this critical real-world application. Compared to regular testing, our experimental results show that the combination of different heuristics leads to significantly higher axiomatization coverage, This translates into a significant increase in trust in the program verification system.