Minor (2mol%) additions of cobalt oxide are highly effective as a sintering additive for the fluorite material Ce0.8Pr0.2O2-delta allowing dense materials to be formed at 1000 degrees C with sub-micron grain size. The addition gives slightly enhanced total conductivity due to an enhanced electronic conductivity of around two to three times, whilst the level and nature of ionic conductivity is unaffected. This produces a decrease in ionic transference number, an increase in the ambipolar conductivity and importantly a decrease in oxygen surface exchange limitations. The combination of these effects boosts the level of oxygen permeability offered by this fluorite material to levels that can compete with some of the best perovskite oxygen permeation membranes at intermediate temperatures, (< 850 degrees C), in a material that offers good redox stability. As such, the material Ce0.8Pr0.2O2-delta containing minor (2 mol%) additions of cobalt oxide offers one of the highest levels of oxygen permeability reported to date for a single component, mixed conducting, fluorite material. (c) 2007 Elsevier B.V. All rights reserved.