Automated service composition has been investigated thoroughly during the last five years. Although it promises to alleviate the difficulties of manual service composition, it will only work if complete and correct service specifications are available.

In this paper, we present a third approach - semiautomated composition - based on three mixed initiative features which we have derived from an industry case study with SAP. These features, filter inappropriate services, check validity, and suggest partial plans, are definded on the basis of a common formal model. Related approaches, in contrast, are limited to supporting individual mixed initiative features. To show the applicability of our approach, we have developed a prototypical implementation. Our results show that our mixed initiative approach significantly eases the modeling of service compositions.