Chlorine-substituted brownmillerite Ba1.95In2O4.9Cl0.1 was obtained from barium indate Ba2In2O5 by solid-phase synthesis. The ability to absorb water from the gas phase was confirmed by thermogravimetric studies. The transport properties were studied while varying the thermodynamic parameters of the external environment (T, pO2, pH2O). The chloride ions in the oxygen sublattice of barium indate Ba2In2O5 were found to affect the ion conductivity. In a humid atmosphere, the sample exhibited proton conductivity (E a = 0.54 eV), whose contribution became dominant below 300°C.