Although the development of service-based software has evolved greatly in the last years, there is still a lack of accurate development methodologies applied to service orientation. To solve this, we focus on the Model-Driven Development approach, and more specifically in MDA, the proposal by the OMG, as it is one of the main trends in nowadays Software Engineering. Accordingly, we propose to define a Service-Oriented Software Architecture (SOSA) model within a model-driven methodological framework called MIDAS, for the adaptable and flexible development of modern systems. In the present work we study the concepts and elements that need to be defined at a platform independent and neutral abstraction level. This paper reflects our proposal of PIM-level architecture model for Service-Oriented Software Architectures within MIDAS.