A two-compartment photoelectrochemical cell consisting of a TiO2 photoanode, Nafion membrane and platinized tin oxide glass as the cathode was constructed. The anolyte and catholyte were 1 M NaOH and 1 M Na2S4, respectively. Illumination of the photoanode resulted in oxygen evolution in the anodic compartment simultaneously with the reduction of tetrasulfide in the cathodic compartment. The mechanism of such a photoelectrochemical oxygen evolution is discussed throughout and the possible application of this reaction in solar water splitting is mentioned. (C) 2004 Elsevier B.V. All rights reserved.