The aim of this paper is to illustrate how formal specifications for collaborative interactive systems might be written. It presents a new modelling paradigm for certain systems. It also shows how formal software engineering approaches can be useful. Specifically we choose to specify a simple collaborative editor. This example serves two purposes: it shows how clear and simple a formal specification can be and it provides a basis for making observations about the requirements for a specification language where the target is CSCW systems. The specification of the system has three parts: the semantics of the system; the syntax of the system; the semantics of the collaborative aspects of the system.