Characterizing the equilibrium between molten (Li2CO3 + Li2O) and CO2 is important for a number of applications ranging from carbonate fuel cells to Li industrial production. The equilibrium pressure of CO2 was measured above (Li2CO3+ Li2O) mixtures containing mole fraction of lithium oxide, x(Li2o). between 0.01 and 0.06 and heated within the temperature range (1073 to 1248) K. These data were used to evaluate the enthalpy and entropy of thermal decomposition of Li2CO3. For 0.02 <= X-Li2O <= 0.06, both values remain constant to within experimental uncertainty: Delta H = (275 +/- 5) kJ.mol(-1) and Delta S = (179 +/- 4) J.mol(-1).K-1. For mole fraction of lithium oxide 0.01 <= X-Li2O <= 0.02, both Delta H and Delta S decrease considerably with decreasing concentration of the oxide. Nevertheless, they remain much larger than the values calculated for the decomposition of Li2CO3 based on thermodynamic quantities reported for the formation of CO2, Li2O, and Li2CO3: Delta H-5 = 148 kJ.mol(-1) and Delta S-s = 79J.K-1.mol(-1). We attribute this discrepancy to the solubilization of Li2O by Li2CO3. Using our derived thermodynamic parameters, we can predict the existence of a range of temperatures and concentrations of Li2O in the Li2CO3 melt that are in equilibrium with atmospheric CO2 or are capable of absorbing CO2 from air. Experimentally, it was verified that, following melting at 1008 K. a Li2CO3 melt is stable in air at 998 K for at least 30 h without signs of Li2O precipitation. The stability of the melt is attributed to partial decomposition of Li2CO3 into Li2O, which decreases the liquidus temperature and reduces the equilibrium partial pressure of CO2. Our findings prompt a revised view of the thermal stability of Li2CO3 melts in air. (C) 2011 Elsevier Ltd. All rights reserved.