Some heterogeneous reactions of oxide ion exchange (carbonate ion dissociation and magnesium oxide dissolution) in the molten {KCl + LiCl} eutectic at temperatures of (873, 973 and 1073) K were studied using an electrochemical cell with an oxygen membrane electrode Pt(O-2)\ZrO2(Y2O3). The dissociation constant of the CO32- was found to increase with increasing temperature: pK (873 K) = (2.39 +/- 0.05); pK (973 K) = (1.81 +/- 0.09); pK (1073 K) = (1.53 +/- 0.08). Removal of CO2 from the gas above the melt allows the complete transformation of CO32- to O2-. pP(MgO) values decrease more from (6.99 +/- 0.08) to (5.41 +/- 0.04). The oxobasicity indices, pI((KCl+LiCl)), were calculated from the solubility data to be 3.2 at 873 K, 3.4 at 973 K, and 3.6 at 1073 K. This trend suggests an increase in acidity with increasing temperature of {KCI + LiCl}. (C) 2004 Elsevier Ltd. All rights reserved.