Reactions of Be2+ and Mg2+ with O2- in molten eutectic mixture (CsCs + KCl + NaCl) (0.455:0.245:0.30) at T = 783 K were studied by a potentiometric method using Pt(O-2)vertical bar ZrO2(Y2O3) indicator electrode. Addition of O2- ions to the melt containing Mg2+ results in precipitation of MgO (pK(s,MgO) = 11.89 +/- 0.3, molality) whereas interaction of Be2+ with O2- is accompanied with sequential formation of Be2O2+ (pK = 15.68 +/- 0.5, molality) and precipitation of BeO (pK(s,BeO) = 9.62 +/- 0.3, molality). On the basis of the obtained and known data pK(s,MgO)-T-1 dependence in molten (CsCs + KCl + NaCl) eutectic is constructed. The slope of the said dependence in T/K = (from 583 to 1073) range is in good agreement with the value predicted by the Shreder equation, that extends the range of use of the Shreder equation for predictions of metal oxide solubilities in molten halides. (C) 2012 Elsevier Ltd. All rights reserved.