The EPR study of optical‐quality AgGaS2has proven the existence of the point defects Ni3+, Fen+, (n⩽2), a shallow donor (ED<0.11 eV), and one associated defect Fe3+‐X, the charge states of which are all photosensitive. The effectiveness of the standard annealing procedures, employed to obtain crystals of low absorption in the 0.5–12‐&mgr;m range, is associated with an oxidation of these transition‐metal impurities.