The oxygen-releasing step of the ZnFe2O4/(ZnO + Fe3O4)-system for solar hydrogen production with two-step water splitting using concentrated solar energy was studied under the air-flow condition by irradiation with concentrated Xe lamp beams from a solar simulator. The spinel-type compound of ZnFe2O4 (Zn-ferrite) releases O-2 gas under the airflow condition at 1800 K and then decomposes into Fe3O4 (= (FeFe2O4)-Fe-II-O-III) and ZnO with a nearly 100% yield (ZnFe2O4 = ZnO + 2/3Fe(3)O(4) + 1/6O(2)). The ZnO was deposited as the thin layer on the surface of the reaction cell wall. A thermodynamic study showed that the ZnO was produced by the reaction between the O-2 gas in the air and the metal Zn vapor generated from ZnFe2O4. With the combined process of the present Study on the O-2-releasing step and the previous one on the H-2 generation step (ZnO + 2/3Fe(3)O(4) + 1/3H(2)O = ZnFe2O4 + 1/3H(2)) for the ZnFe2O4/(ZnO + Fe3O4)-system, solar H, production was demonstrated by one cycle of the ZnFe2O4/(ZnO + Fe3O4)-system, where the O-2-releasing step had been carried out in air at 1800 K and the H-2 generation step at 1100 K. (c) 2005 Elsevier Ltd. All rights reserved.