The framework for automatic mediation of two process models composed of semantically annotated web services is presented. Process mediation is hard because of many possible mismatches between process models. We introduce algorithms for the process models analysis to find possible mappings between provider's and requester's process models, or to identify incompatibilities that cannot be reconciled with given set of available data mediators and external services. Results of the analysis phase are used in the mediator runtime component. In particular, we show how the workflow and dataflow mismatches can be resolved.