The Gibbs free energies of formation of the solid YbMnO3, HoMnO3, and ErMnO3 phases in the (Ln + Mn + 0) systems were determined by employing electrochemical cells with zirconia solid electrolyte in the temperature range (1150 to 1450) K, The results obtained in this study were used to derive the Gibbs free energy of formation of solid phases from the oxides. The respective equations took the following form: for YbMnO3: (Delta(f)G degrees(m) +/- 410)/(J.mol(-1)) = -29,189+0.021(T/K), for HoMnO3: (Delta fG degrees(m) +/-730)/(Jmol(-1)) = -25,324 + 4.265(T/K). for ErMnO3: (Delta FG degrees(m) +/- 480)/(J mol(-1)) = -29,189 + 0.021(T/K). (c) 2006 Elsevier Ltd. All rights reserved.