Research has shown that offshore outsourcing processes may influence the behavior and strategic choices of firms, but little is known about the determining factors that influence the evolution and outcomes of those processes. Furthermore, longitudinal studies able to generate such insights are lacking. This paper suggests a detailed, activity-based approach to the study of the process of offshore outsourcing of high-value, advanced service activities. While earlier research has considered either firm-internal or firm-external sources of resource building, this study offers a more comprehensive theoretical model that combines resource-based theory and international business network theory. It aims to investigate how determinants of the offshore outsourcing process contribute to the resource stocks of client firms. Based on two longitudinal case studies of offshore outsourcing to India, the study finds that offshore outsourcing operations, in general, make positive contributions to the resource stocks of client firms. Some determinants contribute to the building of resources (partnership commitment decisions, knowledge creation and learning, trust building, the interconnectedness of resources) while others impede resource building (time compression diseconomies, lack of resource mass efficiencies). Notably, the interconnectedness among onshore activities, offshore activities and the underpinning knowledge resources reduces the risk of erosion of client firm resources, although this remains a long-term risk.