Investigation of the optical and photolytic properties of large silver‐halide crystals has shown that both cuprous and cupric ions can be incorporated in the AgCl lattice in true solid solution up to concentrations of the order of 1018ions/cm3, and quantitatively measured by their characteristic optical absorption as well as by their role in the photolysis of such crystals. At elevated temperatures, the valence state of the copper is determined by the external chlorine pressure. The dependence, at equilibrium, of the cupric‐ion concentration on the external chlorine pressure has been determined from measurements of the optical absorption of the cupric ion. This experimentally determined dependence has been compared to that expected from a consideration of the equilibrium number of filled copper levels and the known dependence of free‐hole concentration on halogen pressure. Apart from the absorption bands associated with the copper, another absorption band, whose intensity is proportional to the free‐hole concentration, has been observed in both pure and copper‐containing crystals.