In service-oriented systems non-functional properties become very important to support run-time service discovery and composition. Software engineers should take care of them for guaranteeing the service quality in all the software life-cycle phases, from requirements speci cation to design, to system deployment and execution monitoring. This wide scope and the criticality of non-functional properties demand that they are expressed in a language which is intuitive and easy to use for the service quality specifi cation, and at the same time is machine-processable to be automatically handled at run-time. In this paper we present a Property Meta-Model that aims to reach these two main objectives and show as a proof of concept its use for the modeling of two di fferent properties.