The importance of strategic information systems (SIS) in the financial industry is documented in many studies. But still there is a virulent lack of frameworks to explain the profit impact of IT in general and to guide firms in exploiting the IT resource as a source of competitive advantage. By incorporating findings from the resource based view (RBV) and strategic alignment literature we elaborate key concepts potentially leading to a sustained competitive advantage (SCA). Supported by four case studies from the financial services industry, our findings suggest that the exploitation of SIS for achieving SCA requires IT business alignment based on organizational routines of cross-departmental interaction. These concepts are explicitly modeled and integrated into a formal model using microeconomic theory. Especially interactions between the IT and business domain are found to be a key success driver.