The reaction of carbonate ion dissociation in molten Csl and Nal was studied in the temperature range from the melting point to 800 (Csl) and 830 degreesC (Nal) by a potentiometric method with the use of a membrane oxygen electrode as an indicator one. The dissociation constant of CO32- in the molten iodides was found to increase with the temperature. pK values for Csl are 4.47 (650 degreesC), 4.23 (700 degreesC), 3.89 (750 degreesC), 3.44 (800 degreesC); those for Nal are 4.68 (700 degreesC), 4.39 (750 degreesC), 4.17 (800 degreesC), 3.92 (830 degreesC). The plots of pK vs reverse temperature are linear. Carbonate stability in molten Csl is lower than that in the Nal due to lower stability Of Cs2CO3 compared with Na2CO3.