The oxygen and proton conductor Ba2In2O5 is known to undergo a structural transformation in humid atmosphere at about 300 degrees C. The resulting phase Ba2In2O5 . H2O was investigated by in-situ high-temperature X-ray diffraction. During the transformation, a transient structure existing near 320 degrees C in a narrow temperature interval is formed. Ba2In2O5 . H2O is stable from approximate to 300 degrees C down to room temperature and may be characterized by S.G, P4/mmm, No. 123, tetragonal, with a = 4.1827(10) Angstrom, b = 4.1827(10) Angstrom and c = 8.9617(10) Angstrom. The atomic rearrangement during the transformation together with the change of the coordination of one half of the In atoms from tetrahedral to octahedral symmetry reduces the unit cell volume of Ba2In2O5 . H2O to one quarter of the initial value. Linear thermal expansion coefficients of both phases show a strong anisotropy.