The oxygen-releasing step of the ZnFe 2O 4/(ZnO + Fe 3O 4)-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 ZnFe 2O 4 (Zn-ferrite) releases O 2 gas under the air-flow condition at 1800 K and then decomposes into Fe 3O 4 ( = Fe II Fe 2 III O 4 ) and ZnO with a nearly 100% yield (ZnFe 2O 4 = ZnO + 2/3Fe 3O 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 ZnFe 2O 4. 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 3O 4 + 1/3H 2O = ZnFe 2O 4 + 1/3H 2) for the ZnFe 2O 4/(ZnO + Fe 3O 4)-system, solar H 2 production was demonstrated by one cycle of the ZnFe 2O 4/(ZnO + Fe 3O 4)-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.