The perovskite-related tungstate of Ba4Na2W2O11 with oxygen deficiency was studied by X-ray powder diffraction, IR-spectroscopy, electrical conductivity measurements and thermogravimetry. Electrical conductivity and transport numbers were measured with varying T in dry (pH(2)O = 10(-4.5) atm) and wet (pH(2)O = 1.92. 10(-2) atm) atmospheres. This phase is capable of dissociative dissolution of water and can exhibit proton transport below 500 degrees C. The localization of protons on oxygen results in the appearance of OH- groups, and a small fraction of protons is present in the form of H3O+-ions. (c) 2010 Elsevier B.V. All rights reserved.