The business value of Information Systems (ISs) has been claimed and challenged. In particular, achieving competitive advantage through ISs has been predicted but as yet not supported by conclusive evidence. One of the key reasons for the mixed and inconclusive results, it is argued, is conceptual deficiency and the lack of theoretical models that can explain how an IS changes an organisation, so as to impact on its competitive advantage. In this paper we investigate this impact by drawing from a case study of an Investment Banking Company and its core IS. In order to explore and understand the observed changes and the impact on the Company’s competitive advantage, we focus on the coevolutionary effects in the development of the Company and its IS. We argue that by viewing the system’s development from a coevolutionary perspective, it is possible to identify the subtle ways in which the IS affects the Company, its work processes, production of services and relationships with clients, that in turn impact on the Company’s competitive advantage. Moreover, we demonstrate how the IS-Company coevolution framework can assist in exploring future risks in maintaining competitive advantage.