The aim of this paper is to analyze the formation of R&D networks in a setting where spillovers between partners may be imperfect, due to knowledge tacitness, and partner-specific, depending on firms’ technological specializations. We find that firms have strong incentives to use the network to gain a competitive advantage and create (<italic>ex post</italic>) asymmetries. In the three firms case, in which one firm has unique technological capabilities, this firm gains a prominent position in the network, by forming an exclusive alliance and forcing into a weak position the firm outside the network. From a normative point of view, we find that the spillover rates do matter in determining the network that is optimal from the social point of view, and this network structure can be denser than the stable networks. In this case, the optimal policy would consist in <italic>active</italic> support to R&D cooperation.
