Organizations are progressively more required to collaborate with each other in order to achieve their business goals. This had led to the need of integrating the software systems of different, autonomous and geographically distributed organizations. Such integration is increasingly supported by integration platforms which are specialized middleware-based infrastructures that provide an intermediate processing layer, with integration capabilities, between organizations. This paper proposes a reference architecture for integration platforms supporting crossorganizational collaboration. The architecture is presented through four views where the main concepts, use cases and capabilities of these platforms are described.