In this study, we investigated the decomposition of molten Li/Na carbonate under O-2 and N-2 atmospheres through electrochemical methods. Since oxide and peroxide ions produced from the decomposition reaction are electrochemically active, monitoring the relevant current responses led to kinetic information about the thermal decomposition of molten Li/Na carbonate. In linear sweep voltammograms on Au wire electrode, current responses attributed to the oxidation of oxide ions and the reduction of peroxide ions gradually increased with time, and it might be associated with the increasing concentrations of these ions due to decomposition of the molten carbonate. The decomposition rate was roughly estimated to be 1.15 x 10(-6) of decomposed fraction per min, through the theoretical analyses on the peak currents (i(p)) in the potential sweep voltammograms on wire electrode. From the increasing rates of i(p) at various temperatures of molten Li/Na carbonate, the activation energy for the decomposition was determined to be 177.5 kJ/mol. Furthermore, the rates of the formation of peroxide ions and their decay back to oxide ions might be higher than the rate of the decomposition reaction.