Although Service-Oriented Computing (SOC) is a promising paradigm for developing enterprise software systems, existing research mostly assumes the existence of black box services with little attention given to the structural characteristics of the implementing software, potentially resulting in poor system maintainability. Whilst there has been some preliminary work examining coupling in a service- oriented context, there has to date been no such work on the structural property of cohesion. Consequently, this paper extends existing notions of cohesion in OO and procedural design in order to account for the unique characteristics of SOC, allowing the derivation of assumptions linking cohesion to the maintainability of service-oriented software. From these assumptions, a set of metrics are derived to quantify the degree of cohesion of service oriented design constructs. Such design level metrics are valuable because they allow the prediction of maintainability early in the SDLC.