Despite the dominance of the service sector in the last decades, there is still a need for a strong foundation on service design and innovation. Little attention has been paid on service modelling, particularly in the collaboration context. Collaboration is considered as one of the solutions for surviving or sustaining the business in the high competitive atmosphere. Collaborative services require various service providers working together according to agreements between them, along with service consumers, in order to coproduce services. In this paper, we address crucial issues in collaborative services such as collaboration levels, sharing data and processes due to business interdependencies between service stakeholders. Subsequently, we propose a model for Collaborative Service Modelling – the COSEMO model, which is able to cover identified issues. We also apply our proposed model to modelling an example of Travelling services in order to illustrate the relevance of our modelling approach to the matter in hand.