A thermochemical two-step water splitting cycle using a redox system of iron-based oxides or ferrites is one of the promising processes for converting solar energy into clean hydrogen in sunbelt regions. Fe3O4 supported on YSZ (Yttrium-Stabilized Zirconia) or Fe3O4/YSZ is a promising working material for the two-step water splitting cycle. In the water splitting cycle, an iron-containing YSZ or Fe2+-YSZ is formed by a high-temperature reaction between Fe3O4 and YSZ support at 1400 degrees C in an inert atmosphere. The Fe2+-YSZ reacts with steam and generate hydrogen at 1000 degrees C, to form Fe3+-YSZ that is re-activated by a thermal reduction ill a;separate step at 1400 degrees C under an inert atmosphere. In the present work, the thermal reduction was performed in a higher temperature range of 1400-1500 degrees C while the hydrolysis reaction was carried Out at 1000 degrees C. It was confirmed by XRD analysis that the cyclic redox reactions Occurred based on the same reaction mechanism when using a thermal reduction temperature between 1400 and 1500 degrees C. The conversions of Fe3O4 to Fe2+-YSZ were 20, 26 and 47% when the thermal reduction temperature were 1400, 1450, and 1500 degrees C respectively, indicating that the x values in the formed Fe2+-YSZ or Fex2+YyZr1-yO2-y/2+x, were 0.08, 0.11, and 0.19 respectively, where y = 0.15. The conversions of Fe2+-YSZ to Fe3+-YSZ ill the hydrolysis reaction (at 1000 degrees C), however, decreased from 90% to 60% when the thermal reduction temperature increased from 1400 to 1500 degrees C. As the results, the hydrogen production reactivity of Fe3O4 supported on YSZ increased from 5.6 x 10(-4) to 7.5 x 10(-4) g per gram of Fe3O4/YSZ for one cycle on the cycle average by elevated thermal reduction temperature from 1400 to 1500 degrees C.