Organizations adopt IOS (interorganizational systems) to achieve both transactional and strategic benefits. The IOS involves two parties using the system and therefore its success depends on the existing relationship between trading partners. While there are some studies that investigate relationship factors and how they affect adoption, the literature does not specifically highlight that these systems may be implemented in progressive stages over time. This is because these studies typically adopt a cross sectional approach by only taking a snapshot of IOS adoption at single point in time. They do not present an over all picture of adoption decisions and do not explain why and how organizations adopt these systems in a phased approach. In this paper, we investigate IOS adoption by a pair of major organizations in the Australian Grocery Industry over a period of time. We illustrate, using a case study, that a particular level of relationship between the two organizations is required for IOS use and that the initial use of the IOS also affects the relationship which in turn results in more sophisticated IOS adoption. The findings of this study have important implications for both research and practice.