The increased availability of mobile broadband connec- tions enables the expansion of software downloads to mobile devices. This leads to greater number of available services and a better utilisation of the computational power of mobile devices. The downside of this increased software availability is an increase in the possible attack vectors. One scenario is the misuse of resources, for example, a game is sending more SMS's than the user wishes it to send. Thus, techniques need to be in place that prevent downloaded software from misusing resources on the mobile device. In this paper, we show how it is possible to prevent such misuse of resources through the use of a runtime monitor that performs execution-time checks to monitor if the application is behaving correctly or not. This runtime monitor enhances the flexibility of the Java security model for mobile devices and allows to enforce security policies without having to rely on signatures of the downloaded applications.