Water-splitting potential by cation-excessive (Ni, Mn) Ferrite, Ni0.5(1 + epsilon)Mn0.50(1 + epsilon)Fe1.99(1 + epsilon)O-4 was evaluated using the standard Gibbs free energy change (Delta G degrees) for the cation-excessive ferrite formation in different O-2 partial pressures. The cation-excessive degree epsilon ranged from 0.026 to 0.16 in pO(2) values of 7.9 x 10(-7) to 1.0 x 10(-1). From the relation between epsilon value of (Ni, Mn) ferrite and oxygen partial pressure, equilibrium constant K(th) was determined. Furthermore Delta H degrees s for O-2 releasing and water-splitting reactions with cation-excessive (Ni,Mn) ferrite were evaluated from the van＇t Hoff plot and compared with that for magnetite-wustite system; where Delta H degrees s were assumed to be the same values for both (Ni, Mn) ferrite and magnetite-wustite system: +300 kJ for O-2 releasing and -35 similar to -63 kJ for water-splitting. Delta G degrees s evaluated for water-splitting with cation-excessive (Ni, Mn) ferrite and wustite were -38 kJ and -35 kJ, respectively, at 298K. It suggests that water splitting with cation-excessive (Ni, Mn) ferrite proceed easily compared with that with wustite. Delta S degrees s for water-splitting are -0.93 kJ K-1 for the former and -0.83 kJ K-1 for the latter. H-2 generation rates by reaction with H2O for (Ni, Mn) ferrite were studied at temperatures of 573 K-1073 K. It reached the maximum at 1000 K while it proceeds preferentially below 830 K from thermodynamics.