Recent results on radiotracer grain boundary diffusion of different solutes in the same high-purity polycrystalline copper are reviewed. The measurements were performed in extended temperature intervals satisfying Harrison’s B and C regime conditions at higher and lower temperatures, respectively. In the B regime, the triple product P = sδDgb was determined, while the grain boundary diffusivity Dgb was directly measured in the C regime (s is the segregation factor and δ the grain boundary width). Consequently, the segregation of different solutes in the copper matrix was determined for the true dilute limit conditions. The results on grain boundary diffusion and segregation are analysed in relation to the solute – solvent binding and solute – vacancy interaction in the bulk and in the grain boundaries. By increasing amount of the applied radiotracer the effect of solute concentration on grain boundary diffusion can thoroughly be examined. Grain boundary diffusion experiments on well-characterised bicrystals have been shown to be most suitable for such a study. In a radiotracer experiment, the complete solute segregation isotherm can be measured beginning already from a dilute solution in both, bulk and grain boundary.