Modelling is recognised as an essential activity in the architectural design of software systems. Model-driven architecture (MDA) is a framework implementing this idea. Ontologies are knowledge representation frameworks that are ideally suited to support modelling in this endeavour. We propose here a layered ontological framework that addresses domain modelling, architectural modelling, and interoperability aspects in the development of service-based software systems. We illustrate the benefits of ontological modelling and reasoning for service-oriented software architectures within the context of the Web Services.