We describe a method based on UML activities for the unified specification of collaborative service behavior and local user interfaces. The method enables a model-driven development process, which eectively combines the need to express service collaborations involving several components with the need to provide detailed operations for user interfaces. Our service models use activities as the primary building blocks that encapsulate self-contained functionalities. We show, how a complete distributed system can be decomposed into such building blocks, and how this decomposition leads to a natural separation of user interface concerns from service collaboration concerns, and how they may be combined with our event-driven composition mechanisms based on activity parameter nodes. We illustrate our method with a case study of a situated collaborative learning service.