The semantic web promises to bring automation to the areas of web service discovery, composition and invocation. In order to realize these benefits, rich semantic descriptions of web services must be created by the software developer. A steep learning curve and lack of tool support for developing such descriptions thus far have created significant adoption barriers for semantic web service technologies. In this paper, we present a model-driven architecture based approach for specifying semantic web service compositions through the use of a UML profile that extends class and activity diagrams. This profile is used in transformations that facilitate automatic construction of OWL-S specifications from UML diagrams. Conditions required by the composition, such as those on control constructs, are specified using OCL and transformed into SWRL during the construction process.