The solubility of oxide ions in LiCl-rich LiCl-MClx (M = Na, K, Cs, Ca, Sr, or Ba) melts was measured in the temperature range 723-923 K as part of the development of an electrolytic reduction process for spent nuclear fuel because the solubility of oxide ions strongly affects the rate of oxide reduction. With the exception of the LiCl-CaCl2 system, the precipitate was Li2O under an oversaturated condition. The solubility of Li2O in molten LiCl at 923 K was determined to be 11.64 +/- 0.04 mol %. The solubility significantly decreased when alkali metal chlorides (NaCl, KCl, and CsCl) were added to LiCl and slightly decreased when BaCl2 was added. In contrast, the addition of SrCl2 increased the Li2O solubility. Empirical formulas for the relationship between Li2O solubility and salt composition were derived. The logarithm of Li2O solubility plotted against the reciprocal temperature gave an approximately linear relationship in the LiCl-KCl system. The addition of CaCl2 to LiCl saturated with Li2O gave a CaO precipitate. (C) 2010 The Electrochemical Society. [DOI:10.1149/1.3456631] All rights reserved.