Employing electrochemical cells with the solid zirconia electrolyte Cu2O, Cu(2)Ln(2)O(5), Ln(2)O(3) parallel to O-2- parallel to air and Cu2O, CuGd2O4, Gd2O3 parallel to O2- parallel to air the Gibbs free energy of formation of the solid Cu(2)Ln(2)O(5) (Ln=Yb, Tm, Er, Ho, Dy) and CuGd2O4 phases was determined. The results obtained were used to derive Gibbs free energy change of reactions of formation of these compounds from the respective oxides in the following form Delta G(oxides)(0) (Cu2Yb2O5) = 19 429-22.02 TJ mol(-1) Delta G(oxides)(0) (Cu2Tm2O5) = 35 275-34.09 TJ mol(-1) Delta G(oxides)(0) (Cu2Er2O5) = 17 427-19.61 TJ mol(-1) Delta G(oxides)(0) (Cu2Ho2O5) = 18 165-19.49 TJ mol(-1) Delta G(oxides)(0) (Cu2Dy2O5) = 16 648-17.58 TJ mol(-1) Delta G(oxides)(0) (CuGd2O4) = 9562-12.29 TJ mol(-1).