Free energies of the hexagonal phase and perovskite phases of BaCo1-yFeyO3-(delta),a mixed ionic and electronic conductor, are calculated using first principles method. The calculated structure of a known phase is in good agreement with experiment. It is found that the hexagonal phase is most stable at low temperature. As temperature increases, a phase transition to tetragonal perovskite, in which the B site cation valence is reduced to 3+ by release of oxygen, takes place. Further increase of temperature results in the reduction of the perovskite phase, i.e., the valence of the B site cation is reduced to 2+. The temperature of the hexagonal to perovskite phase transition is lowered by mixing Co and Fe, in spite of the increased phonon scattering, due to localization of oxygen vacancies. (C) 2004 Elsevier B.V. All rights reserved.