The synthesis and characterization by XRD and TEM of the compound BaIn0.5Sn0.5O2.75 (BISO) is described. The water uptake was measured quantitatively as a function of temperature and water vapor pressure. For the standard enthalpy and entropy of solution the following values were obtained Delta H degrees=(-0.76+/-0.08) eV, Delta S degrees=(-1.22+/-0.13)x 10(-3) eV/K. Impedance data were obtained for a wide range of accurately known proton contents leading to precise values of the proton mobilities. An equivalent circuit was suggested consisting of two parallel branches, one for proton and the other for electronic conduction. Concentration cell measurements in O-2-H2O and H-2-H2O atmospheres demonstrated the large contribution of electronic conduction in this compound. Similar conclusions were dr awn from fuel cell experiments with BaIn0.5Sn0.5O2.75 as the electrolyte. TEM inspection of BISO treated at 750 degrees C in dry Ar-4%H-2 revealed that the compound is relatively unstable in this atmosphere and slowly decays to various decomposition products. Chemical modelling using the CHEMSAGE database provided a rationale for this instability.