We propose a formal definition of a pervasive service model targeting the very dynamic environments typical of mobile application scenarios. The model is based on a requirement analysis and evolves from existing service definitions. These are extended with pervasive features that allow for the modeling of context awareness, and pervasive functionality needed for the dynamic (re-) composition of services. In order to demonstrate its value, we present an implementation of a pervasive service platform that makes use of the service model.