The oxygen-releasing step of the ZnFe 2 O 4 /(ZnO+Fe 3 O 4 ) system for solar hydrogen production with two-step water splitting using concentrated solar energy was studied in air by irradiation with concentrated Xe lamp beams (solar simulator). The Zn content in the ZnFe 2 O 4 is vaporized in the form of metal zinc, and the oxygen gas is released to generate Fe 3 O 4 in air at 1800 K. The vaporized metal Zn reacts with O 2 in air, and is deposited in the form of a ZnO layer on the reaction cell wall. The stability field of the Fe 3 O 4 phase in terms of the O 2 partial pressure and the temperature shows that the solid product of the Fe 3 O 4 after the O 2 -releasing reaction is stable even in 1 atm of O 2 at 1800 K. Thus, the O 2 -releasing reaction (ZnFe 2 O 4 =ZnO+2/3Fe 3 O 4 +1/6O 2 ) can be applied for obtaining 100% O 2 gas in the two-step water splitting using concentrated solar heat.