The combination of so-called B- and C-type solute grain boundary (GB) diffusion measurements is a unique and reliable tool for the determination of the solute segregation factor for the true dilute limit conditions. A series of such measurements with different solutes in the same high purity Cu material gained comprehensive quantitative information on the solute segregation behaviour. Specially designed radiotracer experiments on bicrystals supplied valuable information on non-linear solute segregation and even a complete solute segregation isotherm could be determined. The strong solute segregation may invoke new effects. The GB wetting transition in the Bi-Cu system was investigated by radiotracer diffusion. A pronounced increase of GB diffusivity was observed in the two-phase (solid + liquid) region of the corresponding phase diagram, with the GB diffusivity of Cu being similar to the diffusion rate in a liquid. Such a GB diffusion enhancement exists even in the single phase (solid solution in Cu) region manifesting the existence of a pre-wetting GB phase transition in this system.