Carbonate ion dissociation equilibrium in molten alkaline halides at 800 degreesC (Cs (+) - and K (+) -based halides) was investigated using a potentiometric cell with a Pt(O-2) \ YSZ membrane oxygen electrode to detect the equilibrium O2-molalities. The pK values obtained were 2.22 +/- 0.06 (CsCl), 1.20 +/- 0.1 (CsBr), 3.44 +/- 0.1 (CsI), 3.2 +/- 0.2 (KCl), 2.4 +/- 0.2 (KBr), 4.4 +/- 0.2 (KI), which showed that the change of oxoacidic properties was not monotonic in the sequences MeCl-MeBr-Mel (where Me = Cs, K) which differ by 1 pK unit. The dissociation constants for the molten caesium halides are higher (by similar to 10 times) than those of potassium halides although the acidities of Cs+ - and K+-based melts of the same anion composition are practically the same (oxide solubility data). The main reason for the phenomenon observed is the different stability of K2CO3 and Cs2CO3 at 800 degreesC.