Reactivities of three metal oxides (Fe2O3, TiO2 and MnO2) with Na2CO3, and of their reaction products with CO2, have been studied to enhance the O-2-releasing step in the two-step water splitting by the MnFe2O4-Na2CO3 system. X-ray diffraction analysis and thermogravimetric measurements showed that the reaction of alpha -Fe2O3 with Na2CO3 (mole ratio=1:1) completed within 10 min at 1073 K to produce NaFeO2 (Fe2O3+Na2CO3--> NaFeO2+CO2). Also, the regeneration of Fe2O3 and Na2CO3 proceeded readily by passing CO2 gas through NaFeO2 (71% yield). TiO2 reacted with Na2CO3 (mole ratio=1:1) at 1073 K for 1 h to form Na8Ti5O14 and Na2TiO3 (93% yield). However, in the reaction of the products with CO2, the starting material (TiO2) was not reproduced at the temperature range from 673 K to 1073 K, but Na4Ti5O12 (having a lower Na content than the form Na8Ti5O14) was formed. In the case of MnO2 with Na2CO3 (mole ratio=2:1), Na0.7MnO2-2.05 and NaMnO2 were produced at 1073 K for 1 h (80% yield), but the reaction between these products and CO2 hardly proceeded.