This work presents the results of a study of the Tm3+/Tm2+ and Yb3+/Yb2+ couple redox potentials vs. Cl-/Cl-2 reference electrode at the temperature range (973 to 1123)K in molten CsCl 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(l) + 1/2 Cl-2(g) double left right arrow TmCl3(l) and YbCl2(l)+ 1/2 Cl-2(g) double left right arrow YbCl3(l) were calculated using the temperature dependencies of apparent standard potentials of the couples E-*(TM)(3+) /(TM)(2+) and E*(Yb3+) /(Yb2+). (C) 2010 Elsevier Ltd. All rights reserved.