BaZr0.9Y0.1O2.95 prepared at very high sintering temperatures was evaluated as to whether it displayed the high water vapor solubility claimed in recent work. A maximum solubility of 0.083 was determined which is close to the theoretically possible value. Thermogravimetry resulted in the following parameters for the solution process: Delta H degrees=(-0.77+/-0.03) eV, Delta S degrees=(-0.9+/-0.1)x10(-3) eV/K, Electron microscopy showed a bimodal size distribution of the grains. Density measurement provided evidence for the vacancies in the O-sublattice in the dry state,The relevant transport numbers were determined electrochemically. Under oxidizing conditions, hole conduction suppresses the proton transport number. Under reducing conditions, BaZr0.9Y0.1O2.95 is a near perfect proton conductor. Oxygen ion conduction was not found to be important. The use of BaZr0.9Y0.1O2.95 under reducing conditions allows the injection of hydrogen into inert gas streams as shown with a PEM fuel cell. DC conductivities are also presented providing a rationale for the electrochemical cell measurements. (C) 2000 Elsevier Science B.V. All rights reserved.