The reversibility of a membrane oxygen electrode Pt(O-2)\ZrO2(Y2O3) and acid-base reactions in molten LiCl-KCl at 700 degrees C have been studied by the potentiometric method. The calibration plot has been found to consist of three regions -with predominant effect of oxygen admixtures, with the region at pO > 2 and (partial derivative E/partial derivative pO) = 2.303RT/2F and one region at pO < 2 with (partial derivative E/partial derivative pO)= 2.303RT/F x F. Solubility products of CoO(pP = 4.43 + 0.11), NiO (pP = 5.34 +/- 0.2), and MgO (pP = 5.87 +/- 0.05) at 700 degrees C have been determined. Acidic properties of Cd2+ and Pb2+ have been found to be completely levelled to those of Li+. The increase of oxide solubilities and shift of the inflection point on the calibration plot as compared with molten KCl-NaCl and CsCl-KCl-NaCl was caused by the increasing of the acidity of the melt studied. The shift of oxoacidity in Li-based melt has been estimated as similar to 3.5 pO units. The possibility of the use of equilibria studied for the determination of acidity indices of molten alkaline halides is discussed.