The basic principles of semiconductor photoelectrochemistry as applied to light to chemical and/or electrical energy conversions are outlined. Special emphasis is on the photoelectrolysis of H 2O to H 2 and O 2. New results are presented which show that platinized n-type SrTiO 3 and KTaO 3 evolve H 2 and O 2 from alkaline aqueous solutions when irradiated with ultraviolet light. The irradiated portions of the metal oxide behave as the photoanode and the dark platinized portions behave as the cathode. The nonplatinized oxides are energetically capable of evolving H 2 and O 2 upon irradiation but the overvoltage for H 2 is too great. The general requirement for the "short-circuit" redox chemistry at the irradiated n-type semiconductor surface is that the dark cathodic reaction occur at a potential more positive than the anodic reaction upon irradiation. The role of the Pt coating is to reduce the H 2 overvoltage to meet this requirement. It is demonstrated that polished n-type TiO 2 and SrTiO 3 are both capable of oxidizing H 2O to evolve O 2 and reducing Cu 2+ to plate out Cu upon irradiation in 0.5 M CuSO 4 (pH = 7). We discuss the importance of "short-circuit" redox chemistry and overvoltage in obtaining opencircuit photopotential in photoelectrochemical cells.