Ba11W4O23 (or Ba-1.75(BaW)O-5.75) has a derivative perovskite (ABO(3)) structure with vacancies ordered in the A-sublattice, but disordered in the O-sublattice, suggesting the potentiality as an oxide ion electrolyte. Electrical conductivities of Ba11W4-xTaxO23-0.5x were characterized as functions of temperature, oxygen activity, water vapor activity, and composition x (=0.1, 0.5, 1.0, 2.0). It has been found that the compound is indeed an oxide ion conductor, over a wide oxygen activity range, with a moderate ionic conductivity, compared with the conventional electrolyte oxides. The conductivity is little dependent on x, but for x = 1 where an appreciable depression of the conductivity occurs. The oxide also exhibits a proton conductivity at temperature below 800 degrees C. (c) 2008 Elsevier B.V. All rights reserved.