The well known oxygen ion and proton conductor Ba 2 In 2 O 5 transforms to a new hydrate-like structure of composition Ba 2 In 2 O 5 .1H 2 O (labelled β here) when exposed to humidity below 300 o C. The water vapor pressure curve over the coexisting α- and β-phases is described by the following thermodynamic parameters: ΔH 0 =-0.89 eV and ΔS 0 =-1.27x10 - 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 Ba 2 In 2 O 5 -water vapor is presented. The new proton-rich phase Ba 2 In 2 O 5 .1H 2 O is compared with recent high-temperature proton conductors.