The mechanism and the optimum conditions for the reduction of residual double bonds in styrene-butadiene rubber (SBR) latex by hydrogenating the polybutadiene in the latex form were studied. The hydrogenation involves a copper ion (II)-catalyzed procedure in which diimide hydrogenation agent is generated in. situ at the surfaces of latex particles by a hydrazine/hydrogen peroxide redox system. The surface density of the copper ion in particle surfaces was found to be a crucially important parameter in controlling the degree of hydrogenation. The distribution of the double bonds in the latex particles after the hydrogenation was found to be dependent on the particle size and the extent of crosslinking in the particles.