The literature on research joint ventures (RJVs) has emphasized internalizing spillovers and cost-sharing as motives for RJV formation. In this paper we develop two additional explanations: product market complementarities and firm heterogeneity. We analyse a model of RJVs with asymmetric firms and differentiated products. We then test these various explanations for RJV formation using data now available through the US National Cooperative Research Act.