Interactive applications ensure the access to various commercial services (mobile phones, reservation systems or telecommunication services) and are increasingly involved in several critical domains such as flight or industrial process control. Therefore, their correctness becomes a very important issue and their development requires thorough validation [3]. Model-based testing of interactive systems requires modeling multiple user behaviors and generating test sequences taking into account the non-determinism of human reactions. A method to test interactive systems based on the synchronous approach has been proposed [1], which requires a partial formal specification of the software user behavior [5]. To make the specification process easier and more affordable to users who are not familiar with formal languages, the use of task trees [4]annotated with event occurrence probabilities has been proposed. Task trees are built at early stages of the application design and describe the interactions between an application and the user and, hence, provide a model of the user behavior. With appropriate formal semantics, this notation can be used to model test scenarios and to automate test data generation. Test scenarios and test data can be produced for the validation of functional properties and of the control aspects of the application [3].