This work presents the results of a study of the Tm3+/Tm2+ couple redox potentials vs. Cl-/Cl-2 reference electrode at the temperature range (833 to 1073) K in molten equimolar NaCl-KCl and NaCl-2CsCl eutectic by direct potentiometric method. Initial concentrations of TmCl3 in solvents did not exceed 5.0 mol%. Apparent standard redox potentials of the couples ETm3+*(/Tm2+) were determined in different solvents. Basic thermodynamic properties of the reaction TmCl2(1) + 1/2Cl(2(g)) double left right arrow TmCl3(1) were calculated. The dependence between the apparent standard redox potentials of the couples ETm3+/Tm2+* and the ionic potential of the solvents was established. (C) 2011 Elsevier Ltd. All rights reserved.