First, the Gibbs free energy of formation of the solid CuSmO2 and CuSm2O4 phases in the (copper + samarium + 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 of formation of solid Sm2CuBaO5 from the oxides. Next, employing electrochemical cells with the single-crystal calcium fluoride as electrolyte: air, CaO + CaF2 \ CaF2 \ CuSm2O4 + Sm2CuBaO5 + BaF2, air; and air, CaO+ CaF2 \ CaF2 \ CuSm2O4 + Sm2CuBaO5 + Sm2O3 + BaF2, air, the Gibbs free energy of formation of solid Sm2CuBaO5 was determined in the temperature range (973 to 1173) K. The results obtained in this study were used to derive the Gibbs free energy of formation of this phase from the oxides in the following form: Delta(f)G(m)degrees/(J.mol(-1)) = -110580 + 41.29.(T/K).