The oxygen ion and proton conductor Ba2In2O5 was investigated by thermogravimetry. Proton concentrations were obtained as a function of temperature and applied water vapor pressure. Two competing thermodynamic models were applied to the solubility data. For both models, proton uptake is governed by: (H2O)(gas) + V-O(..) + O-O(x) double left right arrow 2OH(O)(.). The first one considers a temperature dependent O-vacancy concentration, the second a constant O-vacancy concentration and trapping. The instability of Ba2In2O5 in humid atmospheres below 300 degrees C resulting in a new high proton-content phase HBaInO3 is discussed. The chemical diffusion of water in Ba2In2O5 was measured as a function of temperature. These data are required to assess penetration times of protons into initially dry bulk samples of Ba2In2O5.