This work presents the results of a study of the Tm3+/Tm2+ and Yb3+/Yb2+ couple redox potentials vs. CI-/ Cl-2 reference electrode at the temperature range (823 to 973)K in fused NaCl-KCl-CsCl eutectic by direct potentiometric method. Initial concentrations of TmCl3 and YbCl3 in solvents did not exceed 5.0 mol%. Basic thermodynamic properties of the reactions TmCl2(1) + 1/2 Cl-2(g) <-> TmCl3(1) and YbCl2(1) + 1/2 Cl-2(g) <-> YbCl3(1) were calculated using the temperature dependencies of apparent standard potentials of the couples E-Tm(+)3+/(2+)(Tm) and E-Yb(+)3+/(2+)(Yb) (C) 2011 Elsevier Ltd. All rights reserved.