The structural phase transition of Ba2Fe2O5+delta from monoclinic to cubic in a N-2 atmosphere has been analyzed using simultaneous thermogravimetry-differential thermal analysis and mass spectroscopy (TG-DTA-MS). It has been confirmed that the structural phase transition is reversible, of the first order, endothermic, and involves a mass increase with increasing temperature. From the mass spectrometry, direct evidence that the origin of the mass variation in the phase transition is the variation of oxygen nonstoichiometry has been obtained. From the simultaneously observed TG curves, the variation of delta for the transition from the monoclinic phase to the cubic phase has been evaluated to be about 0.008. (C) 2013 Elsevier B.V. All rights reserved.