The Gibbs free energies of formation of the solid CuNdO2 and CuNd2O4 phases in the (copper + neodymium + oxygen) system were determined by employing electrochemical cells with zirconia solid electrolyte. The results obtained in this study were used to derive the Gibbs free energy changes of reactions of their formation from the respective oxides. Then, stability domains of the ternary oxides as a function of temperature and p(O-2) were calculated. Next, employing electrochemical cells with the single-crystal calcium fluoride electrolyte:air, CaO + CaF2\CaF2\CuNd2O4 + Nd2BaCuO5 + Nd2O3 + BaF2, air,the Gibbs free energy change of the reaction of formation of the solid Nd2CuBaO5 phase was determined in the temperature range (973 to 1173) K. The results obtained in this study were used to derive the Gibbs free energy change of the reaction of formation of this phase from the respective oxides in the following form: (Delta(f)G(m)(0) +/- 4450)/(J . mol(-1)) = -137880 + 65.18 . (T/K). It was found that the calculated enthalpy value followed the trend established so far for the lanthanide 2,1,1 "green phases". (C) 2003 Elsevier Science Ltd. All rights reserved.