Ni-ferrite (NiFe(2)O(4)) is a promising reactive ceramics of the ferrite for the solar hydrogen production by a two-step water splitting process using concentrated solar energy. However, it should be pretreated before H(2)-generation reaction by grinding the Ni-ferrite sintered after the O(2)-releasing reaction to make a fine powder. If the Ni-ferrite and yttria stabilized zirconia (YSZ) form a solid solution between these oxides (YSZ/NiFe(2)O(4) solid solution=YSZ(Ni,Fe)), it is expected that the two-step water splitting process with the Ni-ferrite system can proceed without treatment of the reduced product because of the high thermal stability of the YSZ/NiFe(2)O(4) solid solution. The YSZ/NiFe(2)O(4) solid solution was prepared by calcination of the mixture of the YSZ balls and NiFe(2)O(4) powder at T=1823 K for 1 h, and its reactivity and thermal stability were examined for the two-step water splitting process. During the ten times repetition of the two-step water splitting reaction (T=1773 K for O(2)-releasing, and 1473 K for H(2)-generation) with the YSZ/NiFe(2)O(4) solid solution using infrared imaging furnace, the reactivity for O(2)-releasing and H(2)-generation was kept constant. The molar ratio of the released O(2) gas volume (the average O(2) gas, 1.9 cm(3)/g) and the generated H(2) gas volume (the average H(2) gas, 3.8 cm(3)/g) was nearly 1:2, indicating that the water decomposition process via two steps proceeds. The X-ray diffractometry (XRD) measurement showed that the YSZ(Ni,Fe) keeps the YSZ phase structure during the ten times repetition of the two-step water splitting process. The successive H(2) gas production by the two-step water splitting process was performed (ten times repetition of the two-step water splitting process). From comparative study on the reactivity and the thermal stability for the two-step water splitting reaction among the YSZ/NiFe(2)O(4) solid solution, NiFe(2)O(4) and ZnFe(2)O(4), it is concluded that the YSZ/NiFe(2)O(4) solid solution is superior to the others.