The thermochemical two-step water splitting cycle was examined by using an iron oxide supported oil yttrium-stabilized. cubic zirconia (YSZ) as the working material with a view toward direct conversion of solar high-temperature heat to clean hydrogen energy. In the first step of the cycle, the YSZ-supported Fe3O4 was thermally decomposed to the reduced phase at 1400 degrees C under an inert atmosphere. The reduced solid phase was oxidized back to the original phase (the YSZ-supported Fe3O4) With steam to generate hydrogen below 1000 degrees C. A new redox pair, which is different front the Fe3O4-FeO pair previously examined by others, served as the working solid material on this YSZ-supported Fe3O4. Our new redox reaction proceeded as follows. The Fe3O4 reacted with YSZ to produce an Fe2+-containing ZrO2 phase by releasing oxygen molecules in the first step: the Fe2+ ions entered into the cubic YSZ lattice. In the second step, the Fe2+-containing YSZ generated hydrogen via steam splitting to reproduce Fe3O4 on the cubic YSZ support. This cyclic reaction could be repeated with a good repeatability of the reaction below 1400 degrees C.