In order to examine the impact of adoption of Information Systems (ISs) on the actual performance of firms, we estimated the complementarities among ISs and found contextual factors using a survey dataset of 616 Korean firms. In particular, we introduced various contextual variables and analyzed their effects on the complementarities of firms. Our results indicate that IT stock serves as an important contextual factor for enhancing the complementarity between adoptions of Enterprise Resource Planning (ERP) and Customer Relationship Management (CRM). It was also revealed that while IT stock has a strong positive direct impact on the performance of firms as well as an indirect impact mediated by ISs, other IT-related variables (such as IT organization and use of firewall) generally have a positive impact only when they are combined with the adoption of ISs.