Here we report the synthesis and photo electrochemical properties of super oxides CuYO2.50 and CuYO2.25 prepared from the delafossite CuYO2, respectively, by thermal oxidation at 380 degrees C under O-2-flow and soft chemistry in NaBrO solution (5N). Their applications as catalysts for H-2 evolution upon visible light were investigated. The oxygen insertion was accompanied by partial oxidation of Cu+. For CuYO2.25, the chemical analyses revealed the presence of mixed valent states containing at least formally an equal number of Cu+ and Cu2+. The thermal analysis (TGA) under reducing atmosphere indicates that oxygen is inserted in different crystallographic sites, for CuYO2.25 it exhibits a two-step reduction mechanism with restoration of the parent oxide. In air, CuYO2+x, is thermally stable up to 500 degrees C above which it undergoes irreversible conversion into Cu2Y2O5. They display p-type behavior ascribed to oxygen insertion and the conduction occurs by hopping mechanism between mixed copper valences. Under illumination, the oxides are stabilized by hole consumption reactions involving SO32- and S2- as holes scavengers. The flat-band potentials, lying between 0.17 and 0.26 V-SCE, allow a spontaneous H-2-photo formation. The rate of H-2-evolution is altered by the oxygen insertion and the best photo activity (1.33 mu mol h(-1) mg(-1)) was obtained over CuYO2.25 immersed in S2- solution (0.025 M); CuYO2 is also reported for a comparison goal. Over time, the photoactivity is slowed down because of the competitive reduction of H2O with the final products namely S2O62- and S-n(2-). (c) 2007 Elsevier B.V. All rights reserved.