The results of oxygen nonstoichiometry, delta, measured by means of coulometric technique as a function of oxygen partial pressure, po(2) in temperature range 900 <= T degrees C <= 1050 are presented for GdBaCo2O6-delta with double perovskite structure. Partial molar enthalpy and entropy of oxygen in GdBaCo2O6-delta structure were calculated. Both thermodynamic properties were shown to increase dramatically in the vicinity of the oxygen nonstoichiometry value equal to 1. The po(2), dependences of oxygen nonstoichiometry and the delta dependences of the partial molar properties were found to have inflections when the oxygen content of GdBaCo2O6-delta is equal to 5.0 exactly. The modeling of the defect structure of the double perovskite GdBaCo2O6-delta was carried out by considering different reference states. Only the model based on the cubic perovskite GdCoO3 as a reference state was shown to fit the experimental data on oxygen nonstoichiometry of GdBaCo2O6-delta good enough. Equilibrium constants of the appropriate detects reactions were, therefore, determined. Concentrations of all defect species defined within the framework of this model were calculated as functions of temperature and oxygen nonstoichiometry. Oxygen vacancies were shown to be formed during po(2) diminution in gas environment in the layers of GdBaCo2O6-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) 2009 Elsevier B.V. All rights reserved.