Water splitting with Mn(II) ferrites (MnFe2O4, Mn0.5Fe2.5O4) and CaO at 1273K has been studied. The process accompanies a phase change and a calcium-manganese oxide (Ca-x(Fe,Mn)(y)O-z) is formed. For CaO/MnFe2O4 mole ratios above 3, a single phase of Ca-3(Fe-2.02/3,Mn-0.98/3)(3)O-7.02 with crystal structure of the A(3)M(3)O(8)-type was formed for the MnFe2O4 sample [Mn(II)/Fe = 1:2]. Associated evolution of H-2 (17.8 cm(3)/g) was observed. The chemical composition of the solid produced shows that the H-2, comes from oxidation of Mn(II) to the Mn(III) ion. In the Mn0.5Fe2.5O4 sample [Mn(II)/Fe = 1:5], the crystal structures of the solid products were assigned to types of Ca-3(Fe,Mn)(3)O-8 and Ca2Fe2O5, the chemical compositions of which could not be determined due to the mixed phase. The H-2, volume (21.6 cm(3)/g) was nearly the same as for an MnFe2O4, sample. Thus, the H-2, evolution reaction for the Mn0.5Fe2.5O4 sample involves oxidation of both Fe(II) and Mn(II) ions in ferrite. H-2, evolution due to oxidation of Mn(II) ions in ferrites was confirmed by the fact that the H-2 volume increased in the order NiFe(2)O(4)much less than Ni0.5Mn0.5Fe2O4 < MnFe2O4. The Mn(II) content also increases in this same order.