The phase diagram of the Na2O-MnO-Fe2O3 system forms the basis for thermodynamic consideration of H-2 production in water splitting with the Na2CO3/MnFe2O4/Fe2O3 system. Sodium iron manganese oxides Na0.7-1 (Mn1-x, Fe-x)O2+delta. were observed in the phase diagram at T = 1273 K under P-O2 = 1.23 X 10(-5) atm. Confirmation of this phase, especially for x > 0.5, suggests the possibility of H-2 generation under this oxygen partial pressure and results in a value of 1:37 for the ratio of P-H2:P-H2O in the H-2 generation step of water splitting. This oxygen partial pressure is realized by the decomposition of carbon dioxide (CO2 = CO + 0.5O(2)) and it is concluded that the H-2 generation step can proceed under CO2 atmosphere. (C) 2002 Elsevier Science Ltd. All rights reserved.