In this work the effect on the oxygen dissolution of adding La2O3 to the binary eutectic 52/48 mol % Li/Na carbonate melt has been evaluated in the temperature range 550-650 degrees C. The amount of dissolved O-2 has been determined by accurate chemical titration. The addition of 0.5 mol % La2O3 to the carbonate salt has been found to result in a dramatic increase of O-2 solubility and also in a change of the activation energy (E-a) for the oxygen dissolution process. The experimental values for E-a correspond closely to a peroxide mechanism in the La2O3-added carbonate as compared to a molecular oxygen or a prevalent superoxide in the pure Li/Na carbonate at 600 and 650 degrees C, respectively. For a better understanding of the O-2 dissolution path, a study also has been conducted by O-2 solubility determinations at different CO2 and O-2 partial pressures using a curve-fitting method. The peroxide mechanism has been confirmed at 650 degrees C, while at 600 degrees C simultaneous presence of a percarbonate dissolution path cannot be excluded. Overall, these O-2 solubility results can provide the basis for the explanation of the La electrocatalytic properties on the oxygen reduction reaction at the MCFC cathode. (c) 2005 The Electrochemical Society.