This study analyzes marginal social and private returns of R&D investment through the impact of international spillovers of R&D stocks. We compare the marginal social with marginal private returns using data of 27 OECD and EU countries from 1995 to 2008. We consider two channels of R&D spillovers: embodied in trade flows and disembodied by bilateral technological proximity. We find that marginal social returns on R&D are much larger than the marginal private returns for R&D-intensive countries, in the embodied spillover channel. We also find that the embodied spillover channel through import flows is more important than the disembodied channel.