Web services are essentially black box components from a composer?s or a mediator?s perspective. The behavioural description of any service can be asserted by the composer, only through interface predicates exposed by the service provider. Normally for proving properties of service compositions, pre/post conditions are found to be sufficient. However these properties, are assertions only on the initial and final states of the service respectively. They do not help in specifying/verifying ongoing behaviour of an individual service or a composed system. We propose a framework for enriching service descriptions with two compositional assertions: assumption and commitment that facilitate reasoning about service composition and verification of their integration. The technique is based on Interval Temporal Logic (ITL), a sound formalism for specifying and proving temporal properties of systems.