The hydrogen generation reaction in the H2O/ZnO/MnFe2O4 system was studied to clarify the possibility of whether this reaction system can be used for the two-step water splitting to convert concentrated solar heat to chemical energy of H-2. At 1273 K, the mixture of ZnO and MnFe2O4 reacted with water to generate H-2 gas in 60% yield. X-ray diffractometry and chemical analysis showed that 48 mol% of Mn-II (divalent manganese ion) in the A-site of MnFe2O4 was substituted with Zn-II (divalent zinc ion) and that chemical formula of the solid product was estimated to be (Zn0.58Mn0.42Mn0.39Fe1.61O4)-Mn-II-Fe-III (Mn-III : trivalent manganese ion). Its lattice constant was smaller than that of the MnFe2O4 (one of the two starting materials). From the chemical composition, the reaction mechanism of the H-2 generation with this system was discussed. Since the Mn ions in the product solid after the H-2 generation reaction are oxidized to Mn3+, which can readily release the O2- ions as O-2 gas around 1300 K, the two-step of H-2 generation and O-2 releasing seem to be cyclic. (C) 2003 Elsevier Ltd. All rights reserved.