The interaction among H2O vapor, CO2 and Ba2In2O3 solid was investigated by thermogravimetry-mass spectroscopy (TG-MS) and X-ray diffraction measurements. X-ray diffraction revealed that the crystal structure of as-prepared Ba2In2O5 was brownmillerite-type orthorhombic; however, incorporation of H2O and CO2 into the specimen was also confirmed by TG-MS measurement. Weight decrease observed at around 100 and 300 degrees C in TG curve of as-prepared Ba2In2O5 could be attributed to the secession of H2O by using mass spectroscopy. It was suggested that H2O adsorbed on the surface of the specimen was released at around 100 degrees C, whereas H2O incorporated in the crystal was released from the specimen at around 300 degrees C. The weight of the specimen also decreased at around 700 degrees C, at which temperature CO, secession was confirmed. By annealing the specimen in humid Ar at 250 degrees C, the amount of H2O incorporated in the crystal increased from Ba2In2O5 0.34H(2)O to Ba2In2O5. 0.98H(2)O involving structural transformation from brownmillerite-type orthorhombic system to tetragonal one other than brownmillerite.