Abstract <p>The mechanism of grain boundary diffusion of cobalt in polycrystalline copper has been studied by the method of emission Mössbauer spectroscopy on ⁵⁷Co nuclei. It is shown that cobalt atoms diffuse along the boundaries of copper grains by the vacancy mechanism. The reasons for significant difference in the grain- boundary segregation factors determined by Mössbauer and diffusion studies are analyzed. The effect of interstitial impurities segregation in grain boundaries and near-boundary areas of crystallites on the Mössbauer spectra parameters is discussed.</p>