The reaction mechanism of AeCeO(3) (Ae: Sr, Ba) with CO2 and its thermodynamics were investigated using scanning-type thermogravimetric/differential thermal analysis, TG-DTA, at different CO2 partial pressures, P (CO2), to estimate their potential as CO2 absorbents. The standard Gibbs free energy change, Delta G degrees of SrCeO3+ CO2 <-> SrCO3+ CeO2 and BaCeO3+ CO2 BaCO3 <-> CeO2 were estimated from equilibrium temperature obtained by-the thermogravimetric analyses at different P(CO2). From an Ellingham diagram, the standard entropy change, Delta S degrees, and standard enthalpy change, Delta Eta degrees, of each reaction were determined. The obtained Delta S degrees and Delta H degrees values of the former reaction showed fair agreement with the thermodynamic calculation, whereas those of the latter reaction showed discrepancies from those of the thermodynamic calculations. The Ellingham diagram also suggested that SrCeO3 was superior to BaCeO3 as a CO2 absorbent because the temperature region of SrCeO3 for which the absorption reaction occurred was higher than that for BaCeO3 at constant P(CO2).