The diffusion of gold into the grain boundaries of copper bicrystals was studied for couples of both a continuous and an instantaneous source. The concentration contours along the grain boundaries were measured by means of electron‐probe microanalysis. For the continuous source conditions at 750°C, the ratio of the coefficients of grain‐boundary diffusion and lattice diffusion decreased from 6×105to about 103with increasing gold concentration from 0.5 to 15 at. % gold. The rate of grain‐boundary diffusion decreases even though there is some increase in the rate of lattice diffusion. There was found some contribution of surface diffusion to the couples with an instantaneous or thin source, i.e., 0.5 &mgr; thick. The concentration contours observed in these couples agreed generally with the theoretical solutions.