Research in transactions planning has recognized the evolvement of Web Services as an industry standard to implement transactional business processes. Web transactions are formed by integrating services in an ad-hoc manner. Distributed transaction protocols may be used to ensure data integrity. However, these protocols require some level of coordinated transaction management. Moreover, individual services must be transaction-aware in order to support necessary compensation operations. These assumptions are unrealistic in the case of the Web. We propose a data modeling and contracting framework for Web services that facilitates the verification of data integrity properties in ad-hoc transactions. The proposed framework enables services' integrator to plan their transactions while ensuring data integrity conditions.