Oxygen nonstoichiometry in the Ruddlesden-Popper type oxides LaxSr3 (-) xFe2O7 - (delta) (x = 0, 0.25 and 0.5) was determined by means of the high temperature gravimetry and the coulometric titration. The measurements were carried out in the temperature range between 773 and 1073 K and the P(O-2) range between 10(-40) and 1 bar. The P(02) dependencies of the oxygen nonstoichiometry exhibited typical plateaus at delta similar to 1.0 for Sr3Fe2O7 (- delta), delta similar to 0.875 for La0.25Sr2.75Fe2O7 (- delta), and delta similar to 0.75 for La0.5Sr2.5Fe2O7 - (delta), where the average oxidation state of iron is 3+. In the LaxSr3 (-) xFe2O7 - (delta) series, La0.5Sr2.5Fe2O7 - (delta) showed the smallest oxygen nonstoichiometry and was the most thermochemically stable compound against on P(O-2), temperature, and the La content. Thermodynamic quantities of the partial molar enthalpy of oxygen, h(O) - h(O)degrees, and the partial molar entropy of oxygen, s(O) - s(O)degrees, were calculated from measured nonstoichiometric data. The behavior of oxygen nonstoichiometry and thermodynamic quantities indicates that the ideal-solution-like defect formation is established (La,Sr)(3)Fe2O7 in the oxygen deficient region. (C) 2016 Elsevier B.V. All rights reserved.