Solubility products of CdO (pK(s,CdO) = 6.80 +/- 0.2), ZnO (pK(s,ZnO) = 10.0 +/- 0.5), NiO (pK(s,NiO) = 11.2 +/- 0.2) and EuO (pK(s,EuO) = 13.1 +/- 0.2) in molten Csl at T = 973 K are determined by potentiometric titration of (0.02 to 0.03) mol kg(-1) solutions of the corresponding metal chlorides by strong base (KOH) using a membrane oxygen electrode Pt(O(2))vertical bar ZrO(2)(Y(2)O(3)) as an indicator. On the basis of pK(s,MeO) values, all the oxides studied are referred to practically insoluble in molten Csl. The values of the oxide solubility in Csl melt are lower than the corresponding values in molten alkali metal chlorides. This can be explained by 'softer' basic properties of I(-) as compared with Cl(-) in the frames of the Pearson 'hard' and 'soft' acid-base concept. In the oxide samples studied, the values of the solubility fall with the decreasing cation radius. The correlation between pK(s,MeO) and the polarizing action by Goldshmidt (Zr(Me2+)(-2)) of the cation is practically linear and may be proposed for estimation of the solubility of s- and d- element oxides in molten Csl on the basis of their cation radii. (C) 2011 Elsevier Ltd. All rights reserved.