Despite the importance of knowledge transfer for the success of offshore information systems (IS) outsourcing, only limited research has been conducted to date. This research investigates knowledge transfer to vendors from their client, in terms of learning from client and learning about client. More specifically, it examines the effects of client support and vendors’ learning mechanisms on knowledge transfer, along with the impact of knowledge transfer on performance. Results show that client support is important for both types of knowledge transfer. In addition, it is the deliberate cognitive learning rather than the relative passive learning of interaction experience that promotes knowledge transfer significantly. Furthermore, whereas learning about client has a positive impact on vendors’ cost control, learning from client contributes to project quality if a vendor maintains a high level of interaction experience with the client.