[en] Full text.As we have published earlier, there is a considerable penetration of copper into the CdS in both the grain and the grain boundary regions. It is of technical as well as general scientific interest to study this diffusion process and also to estimate the values of the diffusion coefficient of copper in these two regions during post-annealing treatment under three different ambient atmospheres (air, vacuum and hydrogen). The diffusion of copper has been studied by Auger electron spectroscopy. The results show that at the temperatures frequently used in post-annealing treatment, hydrogen annealing provides a significantly lower ratio of grain boundary diffusion coefficient D_gb to grain diffusion coefficient D_g than the other two cases