Functional imaging studies have identified a network of brain regions associated with theory of mind (ToM); the attribution of mental states to other people. Similar regions have also been observed in studies where people play games that involve either competing or co-operating with another person. Such games are thought to place implicit demands on ToM processes. Co-operation with others has also been shown to elicit brain responses in areas associated with the processing of reward, suggesting that co-operation is an intrinsically rewarding process. In this study, we used a factorial design to assess the interaction between co-operation and the availability of financial rewards in a guessing game. Twelve subjects were scanned with functional magnetic resonance imaging (fMRI) while they performed a guessing game with and without co-operation, and under both these conditions with and without financial reward. The main effect of co-operation was associated with neural responses in theory of mind regions, while the main effect of financial reward was associated with neural responses in reward regions. Critically the response to reward in medial orbitofrontal cortex was significantly enhanced when subjects were co-operating. This suggests that rewards achieved through co-operation are more valuable than rewards achieved alone.