Inter-organizational knowledge transfer is widely considered to be an important source of competitiveness in the extant literature and by innovation policymakers. This paper reports on a study that investigated inter-firm knowledge transfer in research and development (R&D) projects that received public funding. All projects took place in Finland and included university and/or research institute member(s). Based on multiple case studies, the significance of social capital in knowledge transfer was analyzed. Aspects that limited the knowledge transfer were identified and the effects of motivation on collaboration were evaluated. In certain cases, the knowledge transfer between firms was minimal and existed only in formal meetings. Surprisingly, the firms shared their R&D results openly with other network members, but this was not considered particularly beneficial. The knowledge transfer was limited by changes in consortiums, changes in project duration and/or budget, differences in R&D interests, insufficient resources, and ability to achieve R&D goal(s) without collaboration. In cases with active collaboration, several features of social capital in combination with complimentary business goals were perceived as facilitators of knowledge transfer. Based on the results, practical implications for innovation policymakers and project coordinators, and directions for future research, are proposed. •Social capital supports the inter-firm knowledge transfer in R&D projects.•Making knowledge available does not guarantee the transfer of the knowledge.•Shared business interests facilitated collaboration more than similar R&D topics.•Knowledge transfer between firms was weaker than expected.