Crystal structure of the double perovskite GdBaCo1.8Fe0.2O6-delta was investigated by means of "in situ" X-ray diffraction at temperatures between 25 and 800 degrees C in air. Pmmm-P4/mmm phase transition was found to occur in GdBaCo1.8Fe0.2O6-delta at 515 degrees C. The results of oxygen nonstoichiometry, delta, measured as a function of oxygen partial pressure, p(O2). in temperature range 700 <= T, degrees C <= 1025 by means of thermogravimetric technique are presented for this double perovskite. Partial molar enthalpy and entropy of oxygen in the GdBaCo1.8Fe0.2O6-delta structure were calculated. Both thermodynamic properties were shown to increase dramatically in the vicinity of the oxygen nonstoichiometry value equal to 1. The p(O2) dependences of oxygen nonstoichiometry were found to have inflections when the oxygen content of GdBaCo1.8Fe0.2O6-delta is equal to 5.0 exactly. The analysis of the defect structure of the double perovskite GdBaCo1.8Fe0.2O6-delta was carried out using the model based on the simple cubic perovskite GdCoO3 as a reference state. Equilibrium constants of the appropriate defects reactions were, therefore, determined. As a consequence, concentrations of all defect species defined within the framework of the model proposed were calculated as functions of temperature and oxygen nonstoichiometry. Oxygen vacancies were shown to be formed during p(O2) diminution in gas environment in the layers of GdBaCo1.8Fe0.2O6-delta crystal lattice where they are ordered until oxygen nonstoichiometry of the oxide becomes equal to unity afterwards oxygen vacancies are formed randomly in oxygen polyhedrons. (C) 2012 Elsevier B.V. All rights reserved.