The phase transition behavior of single-phase monoclinic Ba2Fe2O5 was investigated under various oxygen partial pressure P(O-2) values. The first-order phase transition involving mass increase and heat absorption was observed at about 900 degrees C by thermogravimetry-differential thermal analysis measurements under N-2 flow with P(O-2) of 10(-4) atm. The crystal structure of the high-temperature phase was revealed to be cubic perovskite with disordered oxide ion vacancies. By TG-DTA and DSC measurements under various P(O-2) values, it was clarified that the transition temperature and the variation in enthalpy, Delta H, at the phase transition decreased with an increase in P(O-2), indicating that the phase transition involved not only the variation in crystal structure but also the uptake of oxygen from a gas phase. The decrease in Delta H at the phase transition with increasing P(O-2) can be ascribed to the increase in the amount of oxygen absorbed at the phase transition. The phase boundary between the monoclinic Ba2Fe2O5 and cubic Ba2Fe2O5+delta phases obtained by DSC measurements at various P(O-2) values can be explained using the variation in Delta H induced by P(O-2). (C) 2012 Elsevier B.V. All rights reserved.