Two-step thermochemical water-splitting using monoclinic ZrO2-supported Fe3O4 (Fe3O4/m-ZrO2) for hydrogen production was examined at high thermal reduction temperatures of 1400-1600 degrees C. After thermal reduction of Fe3O4/m-ZrO2, the reduced sample was quenched in liquid nitrogen, and was subsequently subjected to the water-decomposition step at 1000 degrees C. Quenching of the solid sample was conducted for analysis of the chemical reactions, such as phase transitions, occurring at high-temperature. The hydrogen productivity of Fe3O4 on a m-ZrO2 support and the conversion of Fe3O4 to FeO were significantly enhanced with higher thermal reduction temperatures. The Fe3O4-to-FeO conversion reached 60% when the Fe3O4/m-ZrO2 was thermally reduced at 1600 degrees C. The phase transition of m-ZrO2 support to tetragonal ZrO2 (t-ZrO2) did not occur during the thermal reduction at 1400-1500 degrees C, but it did proceed slightly at 1600 degrees C. Fe ions from Fe3O4 did not enter the ZrO2 lattice during high-temperature thermal reduction. Thus, the Fe3O4 loaded on a m-ZrO2 support can continuously contribute as a Fe3O4-FeO redox reactant for thermochemical water-splitting at high-temperatures of 1400-1600 degrees C. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.