Process-based web service composition is receiving significant momentum as an important strategy to allow enterprise collaboration. However the current web service composition solutions are rather restricted and inflexible as they use a pre-defined model of the process environment. These solutions have assumed that the information in the models and consequently the compositions remain static and accurate throughout the service composition life cycle. The reflective framework presented here aims to improve the adaptability of BPEL based web service composition. A meta-model was defined to represent the control and data flow of the composition model. To ensure the correctness of dynamic adaptation, a set of constrains and a verification algorithm were defined. A prototype adaptive service composition environment has been developed to implement our solution and demonstrate its effectiveness. In summary, it is stated that the reflective framework provides an considerable solution to the adaptive service composition without causing any control or data flow errors.