Nonstoichiometric variation of oxygen content in La2-xSrxNiO4+delta (x = 0, 0.1, 0.2, 0.3. 0.4) and decomposition P(02) were determined by means of high temperature gravimetry and coulometric titration. The measurements were carried out in the temperature range between 873 and 1173 K and the P(02) range between 10(-20) and 1 bar. La2-xSrxNiO4+delta showed the oxygen excess and the oxygen deficient compositions depending on P(O-2), temperature, and the Sr content. The value of partial molar enthalpy of oxygen approaches zero as delta increases in the oxygen excess region, which indicate that the interstitial oxygen formation reaction is suppressed as delta increase. The relationship between delta and logP(O-2) were analyzed by two types of defect equilibrium models. One is a localized electron model, and the other is a delocalized electron model. Both models can well explain the oxygen nonstoichiometry of La2-xSrxNiO4+delta with a regular solution approximation. (C) 2009 Elsevier B.V. All rights reserved.