[en] Possible reasons of the photocurrent decrease under illumination are discussed by comparison of all experimental data obtained in the previous studies of the photosensitivity degradation in CdS:Cu single crystals. By further studies a photosensitivity decrease is found to be due to vanishing of the shallow donors which compensate the r-centres. A degradation mechanism consisting in transformation of the shallow donors into rapid recombination s-centres, due to their association, is suggested. On heating in darkness these s-centres dissociate to recover both, the initial density of the shallow donors and the high photosensitivity. (author)