Oxygen nonstoichiometry in the Ruddlesden–Popper type oxides LaxSr3 − xFe2O7 − δ (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(O2) range between 10− 40 and 1 bar. The P(O2) dependencies of the oxygen nonstoichiometry exhibited typical plateaus at δ ~ 1.0 for Sr3Fe2O7 − δ, δ ~ 0.875 for La0.25Sr2.75Fe2O7 − δ and δ ~ 0.75 for La0.5Sr2.5Fe2O7 − δ, where the average oxidation state of iron is 3 +. In the LaxSr3 − xFe2O7 − δ series, La0.5Sr2.5Fe2O7 − δ showed the smallest oxygen nonstoichiometry and was the most thermochemically stable compound against on P(O2), temperature, and the La content. Thermodynamic quantities of the partial molar enthalpy of oxygen, hO − hO°, and the partial molar entropy of oxygen, sO − sO°, 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)3Fe2O7 in the oxygen deficient region.