Empirical evidence shows that many firms cooperate in research mainly to gain access to complementary skills. This reality has remained largely unexplored in theory. This paper develops a model that explicitly captures differences in firms’ R&D skills to examine partner choice for R&D cooperation in a triopoly setting. First, we build a general framework to understand how incentives to invest in R&D depend on firms’ skills within the industry. We then analyze coalition stability and partner choice in the presence of high R&D costs, as is often the case when firms cooperate. Our results highlight the role of spillover effects. When spillovers are low, firms prefer to partner with the most competent firm to maximize the gains from cooperation. However, when spillovers are high, the two least competent firms find it optimal to collaborate, as they benefit more from knowledge sharing. Finally, we conduct numerical simulations to assess social welfare, identifying socially optimal coalitions for different levels of spillovers. Overall, our results emphasize the key role of R&D skills and spillovers in shaping both private and social outcomes of collaborative R&D.