CaO solubility in equimolar molten salts CaCl2-x (x= 0, NaCl, KCl, SrCl2, BaClZ and LiCl) was determined at 873- 1223 K and activity coefficient calculated. CaO solubility in the binary salts is less than in CaCl2, and the activity coefficient is greater than one. With increasing temperature CaO solubility increases and the activity coefficient decreases. The dependency of CaO activity coefficient on tempera ture in equimolar molten salts CaCl2 -x is CaCl2 RTIn gamma CaO =6961 +5.06 T (K) 1123-1223 CaCl2-NaCl RTIn gamma CaO =3985 + 17.67 T (K) 923-1123 K CaCl2-KCl RTIn gamma CaO =2384+22.72 T (K) 1073-1223 K CaCl2-SrCl2 RTIn gamma CaO =27245-1.13 T (K) 1073-1223 K CaCl2-BaCl2 RTIn gamma CaO =17068 + 10.19 T (K) 1223-1273 K CaCl2-LiCl RTIn gamma CaO =14724+0.72 T (K) 923-1073 K (C) 2008 Elsevier B.V. All rights reserved.