The small band gap CuAlO2, as prepared, is a p-type semiconductor with a low mobility polarons. Its catalytic ability kfor H-2 production upon visible light was studied in connection with its synthesis method. The potential of the conduction band (- 1.63 V-sce) lies below H2O/H-2 level leading to a spontaneous H-2-evolution. An enhancement has been made in the preparation of active oxide by coprecipitation. The highest H2 production, with a generation rate of 0.19 ml h(-1) mg(-1), has been obtained in a 0.1 M S2- solution at pH 13.72 and increases parallel to polysulfide S-n(2-) formation (nS(2-) + 2(n - 1) H2O -> S-n(2-) + (n - 1) H-2 + 2(n - 1) OH-); both reactions occur simultaneously and no photoactivity was observed below pH 7 indicating that S2- is a determining specie. The carriers' exchange between CuAlO2 and electrolyte occurs iso-energetically; the exchange is possible through induced band gap states acting as efficient relays. The global reaction is governed by the arrival rate of electrons to the interface and the thermal improvement of photoactivity is attributed to an increase in electronic mobility. The decrease of the rate of H-2-evolution over time results from produced S, that act as light filter and interfere with H2O reduction. (c) 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.