The well known oxygen ion and proton conductor Ba2In2O5 transforms to a new hydrate-like structure of composition Ba2In2O5 . 1H(2)O (labelled beta here) when exposed to humidity below 300 degrees C. The water vapor pressure curve over the coexisting alpha-and beta-phases is described by the following thermodynamic parameters : Delta H-0 = -0.89 eV and Delta S-0 = -1.27 x 10(-3) eV/K, X-ray powder diffraction also provided evidence for the above phase transition. DTA experiments showed a large endothermic peak at the transition temperature. A tentative quasi-binary phase diagram of the system Ba2In2O5-water vapor is presented. The new proton-rich phase Ba2In2O5 . 1H(2)O is compared with recent high-temperature proton conductors.