A high efficiency (12. 7%) CdSe//0//. //6//5Te//0//. //3//5/cesium polysulfide photoelectrichemical cell is demonstrated in this work. Crystals of the ternary alloy Cd(Se, Te) of type n were synthesized by the traveling heater method. The nature of the polysulfide electrolyte, based on Cs polysulfide without excess hydroxide, with optimum sulfur and sulfide concentrations, and containing small amounts of copper ions, is shown to be of particular importance in determining the cell efficiency. Reasonable output stability of the cell was obtained under accelerated tests.