Activity coefficients of oxide ion were determined by the measurements of the standard formal potential of O-2/O2- with a boron-doped diamond (BDD) electrode in molten (LiCl + KCl) LiCl:KCl = {58 5,41 5 (eutectic), 65 35, 70:30, 75 25} mol% at T = (673 to 803) K. The activity coefficient decreases with the increase of the LICI content in the melt: (18 7 +/- 1.5, 77 +/- 1.0, 41 +/- 0 4, and 1 6 +/- 0 1), respectively, at T = 773 K in each melt The result is explained by the attractive force between Li+ and O2- which is stronger than that between K+ and O2- (C) 2010 Published by Elsevier Ltd