Generating value in R&D alliances requires intensive and fine-grained interaction between collaborating partners. At the same time, more intensive co-operation increases the risk of competitive abuse of the R&D alliance by one or more partners. In this study, we explore how managers address the fundamental tension between the need for co-operation and the risk of competition, using an in-depth case study of five R&D alliances in the advanced materials industry. Based on our data, we identify two relational strategies to enhance co-operation between engineers of different partners (i.e., adopting boundary-spanning activities and installing similar technical equipment) and three structural strategies to mitigate the risk of such intensified co-operation (i.e., definition of partner-specific task domains, definition of partner-specific knowledge domains and definition of partner-specific commercial domains). In addition, we find that partners tend to use particular combinations of such relational and structural strategies at different stages of the alliance life-cycle to address the co-operation–competition dilemma.