The thermodynamic stability of Ln(III) (Ln: La, Ce, Pr, Tb, and Er) and U(III) complexes in LiCl-KCl system at 923 K was studied by electrochemical techniques. The Gibbs free energy change of the Ln(3+)/Ln(0) and U3+/U-0 redox reactions, Delta G(3/0)(o)', was investigated under various mole fraction of KCl, chi(K) = 0-0.65. In general, Delta G(3/0)(o)' increased linearly with the decrease of chi(K) due to the distortion of octahedral symmetry of LnCl(6)(3-) and UCl63-, but a few exceptions were found for lighter Ln elements, e.g., La and Ce, at smaller chi(K) of 0-0.2. The discrepancy suggests an increase of the number of Cl- ions around La3+ and Ce3+. (C) 2012 Elsevier B.V. All rights reserved.