The O-2-releasing reaction under the air with the reactive ceramics of CeO2-ZrO2 oxides which can be applied to solar hydrogen production via a two-step water splitting cycle using concentrated solar thermal energy was investigated. CeO2 ZrO2 oxides were synthesized by polymerized complex method at different Ce:Zr molar ratio. The solid solubility of ZrO2 in fluorite structure of CeO2 was in good agreement with the initial content of Zr ions at the preparation in CeO2 ZrO2 oxide. The O-2-releasing reaction in air with CeO2-ZrO2 oxides was studied. Different solid solubility (0%, 10%, 20%, 30%) of ZrO2 in CeO2 were examined. The amount of O-2 gas evolved in the reaction with Ce1-xZrxO2 (0 <= x <= 0.3) solid solutions was more than that with CeO2, and the largest yield of 2.9 cm(3)/g was exhibited at x = 0.2 (Ce0.8Zr0.2O2) for an O-2 release at 1500 degrees C in air. The reduced cerium ion in Ce0.8Zr0.2O2 was about 11%, which is seven times higher than that with CeO2. The optical absorption and luminescence spectra of the CeO2-ZrO2 oxide obtained before and after the O-2-releasing reaction suggest that the reduction of Ce4+ with formation of oxygen defect in the air. The enhancement of the O-2-releasing reaction with CeO2-ZrO2 oxide is found to be caused by an introduction of Zr4+, which has smaller ionic radius than Ce3+ or Ce4+, in the fluorite structure. (C) 2011 Elsevier Ltd. All rights reserved.