The oxygen nonstoichiometry of La 2؊x Sr x CuO 4؊ (x ؍ 0-0.3) was measured as a function of Sr content, temperature (400-1000°°C), and oxygen partial pressure (P(O 2 ؍) 1-1؋10 ؊10 atm) using high-temperature gravimetry and coulometric titration. The oxygen nonstoichiometry ranges from oxygen excess to oxygen deficiency one depending on P(O 2 ) and the Sr content, x. Oxygen excess was observed for specimens with x less than 0.05. The dependence of oxygen excess nonstoichiometry on oxygen partial pressures was found to be explained by a model with interstitial oxygen as a predominant defect. In oxygen-deficient regions, partial molar enthalpy and partial molar entropy of oxygen were calculated from the nonstoichiometry data. It was revealed from the variation in partial molar enthalpy that a strong interaction between the oxygen and its vacancy exists in oxygen-deficient La 2؊x Sr x CuO 4؊ . The experimentally obtained partial molar entropy of oxygen was compared with those calculated assuming a so-called metal model, a hopping conduction model, and a narrow band conduction model, where the increase in oxygen vacancies hardly influences the carrier concentration, the holes generated by oxidation of the specimen are trapped by Cu ions, and the holes generated are itinerant, respectively. The variation in partial molar entropy of oxygen could be explained well by either the hopping model or the narrow band conduction model. The oxygen partial pressures required for the decomposition of La 2؊x Sr x CuO 4؊ were also measured through the nonstoichiometry measurement. Discontinuity was observed in the dependence of oxygen partial pressures for decomposition on Sr content between x ؍ 0.05 and 0.10, suggesting an abrupt variation in the thermodynamic behavior of La 2؊x Sr x CuO 4؊ with the Sr content in this region.