While insufficient flexibility of an information system to support a given business process precludes system use in certain cases, excessive flexibility can limit system usability (Silver 1991), in addition to presenting an unnecessary investment. Despite a wealth of research on flexibility and its impacts on organizations and business processes (esp. manufacturing), the value of flexibility and the price at which it comes have rarely been included into the analysis, with the result that guidelines to determine an appropriate (let alone optimal) level of flexibility of organizations, business processes or information systems have not been developed. To support decisions regarding information system flexibility, the current paper presents a conceptual model that determines the performance of a given business process during the lifetime of a supporting information system, with performance measured as costefficiency. The focus of the model is on the trade-off between investments in two types of flexibility: flexibility to use and flexibility to change an information system. After presenting the conceptual model, directions for further research are pointed out.