The depth-concentration profiles of nickel upon diffusion in submicrocrystalline (SMC) molybdenum processed by severe plastic deformation (SPD) have been studied by Auger electron spectroscopy. The coefficients (Db) and activation energies of the grain-boundary diffusion of nickel in SMC molybdenum were determined in a 973–1123 K temperature interval. The results indicate that a difference between the Db values in SMC and coarse-grained molybdenum is related to a nonequilibrium state of grain boundaries in the SPD-processed metal.