Migration of form based legacy systems towards service-oriented computing is a challenging task, requiring the adaptation of the legacy interface to the interaction paradigm of Web services. In this paper, a wrapping methodology is proposed to make interactive functionalities of legacy systems accessible as Web Services. The wrapper that is used for interacting with the legacy system acts as an interpreter of a Finite State Automaton that describes the Model of the Interaction between user and legacy system. This model is obtained by black box reverse engineering techniques. A migration process and a software architecture that allow a functionality of a legacy system to be exported as a web service are presented in the paper.