The functional decomposition of a business process breaks it down into progressively less granular activities. Decomposition contributes to the modular design of a system, the reuse of its parts and to its overall comprehensibility. But achieving these qualities requires a business process to be decomposed consistently, which implies it is always split into an identical set of activities according to a specific purpose, regardless of the modeller’s and modelling context. This paper describes an application of the principle of role-based separation of concerns to consistently decompose a business process into its constituent atomic activities, thus separating its distinct features and minimizing behaviour overlap. An activity is abstracted as collaboration between role types that are played by entities. The decomposition method successively separates the overlapping roles until an activity is specified as a collaboration of an orthogonal set of role types. The method facilitates the consistent decomposition of a business process and the identification of its atomic activities. The relevance of the method is assessed through a number of scenarios according to the guidelines of design science research.