This paper presents a method to analyse the consistency and completeness of process models according to the principles of the ψ-theory and the underlying concept of business transaction. Transactions specify the collaborative behaviour between actors while services are being requested and provided. The method assesses the consistency of a process in terms of the business transactions that can be inferred from it. To do so, it takes as input a process model that is converted to a transactional model. The transactional model is then analysed and revised so that all transactions become consistent and complete according to the transactional pattern. This enables to identify the problems on the original process model and to prompt areas of improvement.