Reactions of Me2+ cations with O2− in molten (KBr+NaBr) (0.5:0.5) and (KBr+LiBr) (0.4:0.6) mixtures at T=973K were studied by potentiometric titration method using Pt(O2)|ZrO2(Y2O3) indicator electrode. In the former melt, the set of cations available for investigation was wide enough (Me=Sr, Ca, Mn, Co, Ni and Pb) and the corresponding solubility product indices (pKs,MeO, molalities) were found to be (3.81±0.3) (SrO), (5.00±0.3) (CaO), (7.85±0.3) (MnO), (8.80±0.1) (CoO), (9.72±0.04) (NiO) and (5.20±0.3) (PbO). A correlation between pKs,MeO and the polarisation action of the metal cation by Goldschmidt (ZerMe2+-2) was obtained. The oxide solubilities on the mole fraction scale were shown to be close to those obtained in molten (KCl+NaCl) equimolar mixture. On the basis of the solubility data the oxobasicity index (primary medium effect for oxide ion) for the (KBr+NaBr) melt was estimated as pI{KBr–LiBr}=−0.24. Due to considerable oxoacidic properties of Li+ cation in the molten (KBr+LiBr) eutectic, it was possible to study only three from the above cations (Me=Mn, Co and Ni) and their pKs,MeO values were (4.36±0.2), (5.19±0.05) and (6.25±0.03), respectively. Comparison with the corresponding data for (KBr+NaBr) equimolar mixture showed that the Li+-based melt dissolved all the studied oxides in more extent. On the basis of the data obtained, the oxobasicity index value pI{KBr−LiBr} was estimated as (3.01±0.5), that was close to the similar parameter of the chloride analogue (KCl+LiCl) (3.36). The change of the constituent anion of the melt in Cl−→Br− does not result in appreciable changes of the melt acidity. Oxoacidic properties of the bromide melts rise in the following sequence: (2CsBr+KBr), pIL=−1.15→(KBr−NaBr), pIL=−0.24→(KBr−LiBr), pIL=+3.01.