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Grain boundary diffusion of 57Co in high-purity polycrystalline copper is investigated using the radiotracer technique in Harrison's B- (850–1150 K) and C-type (550–950 K) kinetic regimes. The triple product P = s·δ·Dgb (s is the segregation factor and δ the grain boundary width) and the grain boundary diffusion coefficient Dgb of Co in Cu are determined to obey the Arrhenius laws with the activation enthalpies of Qgb = 66.2 kJ/mol and Hgb = 100.9 kJ/mol, respectively. Using the experimental estimate of δ, nm, Co is found to segregate strongly at Cu grain boundaries and the corresponding segregation factor follows an Arrhenius dependence with the segregation enthalpy of Hs = −34.7 kJ/mol. Co-diffusion experiments with the 57Co and 110mAg isotopes support a ‘sub-interface’-type of grain boundary segregation of Co in Cu.