In the last years, rich Internet applications (RIAs) have emerged as a new generation of Web applications offering greater usability and interactivity than traditional ones. At the same time, RIAs introduce new issues and challenges in all the Web application lifecycle activities. As an example, a key problem with RIAs consists of defining suitable software models for representing them and validating reverse engineering techniques for obtaining these models effectively.This paper presents a reverse engineering approach for abstracting finite state machines representing the client-side behaviour offered by RIAs. The approach is based on dynamic analysis of the RIA and employs clustering techniques for solving the problem of state explosion of the state machine. A case study illustrated in the paper shows the results of a preliminary experiment where the proposed process has been executed with success for reverse engineering the behaviour of an existing RIA.