A new photochemical system based on a CuYO2 dispersion in aqueous or 3 solution as hole scavengers is proposed. The delafossite CuYO2, doped with calcium, is a low p-type semiconductor with a hole mobility of 7.3 x 10(-7) m(2) V-1 s(-1). The band gap is 3.50 eV and the transition is directly allowed. The valence band edge, located at 5.23 eV below vacuum, is made up from Cu-3d type typical of delafossite oxides. The flat band potential V-fb (+ 0.17 V-sce) lies below the H2O/H-2 potential permitting a spontaneous H-2-formation under band gap illumination in aqueous S2- or SO32- electrolytes. p-CUYO2 loaded with n-CU2O exhibited a higher performance with a H-2 evolution rate of 0.4 cm(3) h(-1) in 0.1 M S2- (pH 13.4). The decrease of photoactivity over time is due to the formation of yellow polysulfides S-n(2-) which compete with H2O in the reduction process. (c) 2005 Elsevier B.V. All rights reserved.