This paper empirically analyses the determinants of firm participation in Research Joint Ventures (RJVs). A review of the theoretical literature highlights the difficulty of identifying testable hypotheses. Using a large database of European RJVs, we estimate a participation equation at the firm level using the logit procedure. We find that sectorial R&D intensity, industry concentration, firm size, technological spillovers, and past RJV participation positively influence the probability of forming RJVs. By contrast, patents’ effectiveness reduce the likelihood of RJV formation. Last, country fixed effects suggest that firms from larger countries are less likely to participate in cross–border RJVs.