The solubility of oxide ions in LiCl-based and CaCl2-based melts was estimated by double-tracer technique as part of the CO2 capture and conversion process. The solubility of oxide ions in pure LiCl and CaCl2 is 10.6 mol% at 600 degrees C and 12.5 mol% at 800 degrees C, respectively. The solubility of oxide ions increases with escalating temperature. The solubility of oxide ions encounters serious decrease when NaCl or KCl is added to the melts. In the case of LiCl-CaCl2 eutectic, the solubility of CaO firstly descends and then ascends with the increasing mole fraction of CaCl2. Intriguingly, the excess addition of Li2O into alkali carbonates leads to a floating layer while, on the contrary, the addition of Li2O into chlorides gives a Li2O precipitate. It is deduced that the Li2O solubility in Li-Na-K carbonates (43.5:31.5:25 in molar ratio) is slightly lower than 5 mol% at 500 degrees C. (C) 2016 The Electrochemical Society. All rights reserved.