In this paper, we argue for the need to thoroughly understand business-to-business (B2B) interaction when designing inter-organisational systems (IOS). This understanding is critical since an IOS both has influence on and is influenced by existing business logic. As a means to reach such understanding we propose a phase model for business interaction, together with a perspective based on business action theory (BAT). Business interaction is defined as consisting of business communication and exchange of value. BAT divides business processes into six generic phases encompassing generic, interactive actions between a seller and a buyer. Different types of exchange occur in these phases. The BAT phase model is an instrument for analysing interaction in a business relation. The outcome of this analysis can then be used for improvements of interaction and serve as a ground for developing a suitable IOS. We explain the theoretical background of this model, i.e. the business action theory, and discuss how the model can be useful in IOS and business design situations. The purpose of the paper is to emphasise that inter-organisational development processes, including IOS design, are depending on a thorough understanding of business interaction.