Nanopowder of Ce0.8Tb0.2,O2-delta, a fluorite-type mixed conductor stable in wide p(O-2) range, was successfully synthesised by the hydrothermal method. Dense ceramics sintered at relatively low temperatures (900 and 1200 degrees C), by employing minor additions of cobalt oxide sintering additive (2 mol%), were compared to those sintered at 1500 degrees C without additives. The effect of the sintering temperature on oxygen transport was analysed in O-2 and N-2 atmospheres. Oxide-ion transference numbers were determined by the modified electromotive force ([ME) method under oxygen/air and nitrogen/air gradients, showing positive temperature dependencies for all sintering temperatures. The partial ionic and electronic conductivities, calculated from the transference numbers and total conductivity, both increase with cobalt additions. A detailed transmission electron microscopy (TEM) study shows a grain boundary location of the cobalt sintering additive at the lowest sintering temperature, 900 degrees C. On increasing sintering temperature the grain-boundary concentration of cobalt is depleted, leading to the presence of segregated grains of cobalt oxide. This factor is shown to be highly relevant with respect to measured oxygen permeation fluxes. Ce0.8Tb0.2O2-delta ceramics sintered at 900 degrees C show significantly higher oxygen permeation, related to improved surface exchange due to the grainboundary enrichment of cobalt and larger grain-boundary area. (C) 2014 Elsevier B.V. All rights reserved.