Software as a Service (SaaS) is a new delivery model for software. Software in a SaaS model is no longer run exclusively for one customer at a customer’s premise but run at a service provider and accessed via the Internet. A provider of Software as a Service exploits economies of scale by hosting and providing the same application for several different customers. However, each individual customer has different requirements for the same basic application. In order to allow each customer to customize the process layer and related artifacts of a SaaS application to their specific needs the application needs to provide a set of variability points that can be modified by customers. In this paper we describe the notion of a variability descriptor that defines variability points for the process layer and related artifacts of process-based, service-oriented SaaS applications. Furthermore we describe how these variability descriptors can be transformed into a WS-BPEL process model that can then be used to guide a customer through the customization of the SaaS application.