We explore the transition phase in information system (IS) offshore outsourcing, focusing on ongoing decisions made in a client-vendor arrangement and the implications these decisions have for knowledge at risk. Drawing on time-compression and knowledge at risk literatures we argue that firms will need to make ongoing transitional decisions over time as knowledge intensive work is increasingly handed over from client to vendor. We conduct a longitudinal exploratory case study of a strategic development project transitioned from a European client to a company in India over a 4-year period. Analysis indicates: (1) four distinct phases of the transition: penetration, embedment, ramp-up and stabilization; (2) how transitional decisions and knowledge at risk vary across these phases. Results from the longitudinal approach have implications not only for understanding the ‘what’ and ‘how’ in IS offshore outsourcing but also for a deeper insight into the ‘when’ question.