The solubility of O-2 in Li-K and Li-Na eutectic carbonate melts has been estimated in the range 600-700degreesC using a highly precise chemical titration method. In general, oxygen solubility is found to be distinctly higher in the Li-K than in the Li-Na eutectic with a tendency for such a solubility difference to disappear at 600 degreesC. The precise determination of O2 solubility values at different O-2 and CO2 partial pressures allowed evaluation of plausible oxygen dissolution mechanisms by a data fitting procedure. Our results support the assumption of a predominant superoxide formation at 650 and 700 degreesC in both melts, with the difference that in Li-Na carbonate oxygen appears to dissolve also through formation of peroxymonocarbonate ions. At 600 degreesC, molecular oxygen is found to be the major oxygen component. (C) 2004 The Electrochemical Society.