Abstract: Both the open circuit potential and stability of an illuminated photo-electrochemical electrode depend on the composition of electrolytic solution in which the electrode is immersed. The open circuit potential varies linearly with the logarithm of light intensity. The potentials of n-type semiconductor photoelectrodes shift to more positive values as the concentrations of oxidants in solutions increase, but they are almost unaffected by the concentrations of the reductants. Formula of photopotential was derived to explain these phenomena. Hysterisis of photopotential was observed in certain solutions. This phenomenon is explained by the split of Fermi-levels within surface layers of illuminated photoelectrodes.