Ce0.9Gd0.1O1.95 (CGO) nanophase materials and related porous supported membranes were synthesized by a soft-chemistry route. One of the membranes was post-impregnated with Pt nanoparticles. The CGO average crystallite size is in the range 9-22 nm and the specific surface area varies from 43.4 to 6.7 m(2)/g in the intermediate temperature domain between 500 and 700 degreesC. The gas permeation properties of Pt-free and Pt post-impregnated CGO porous membranes were studied up to 500 degreesC. Although, the intrinsic bulk ionic conduction properties of the CGO material cannot be expected to contribute to oxygen transport at such a low temperature, attractive surface diffusion effects have been evidenced, which were clearly enhanced by a dispersion of Pt particles. The observed inversion of O-2/N-2 selectivity (alpha > 1) compared to the Knudsen selectivity alpha(K)(O-2/N-2) = 0.935 was attributed to the effect of the nanophase CGO material. Insertion of Pt nanoparticles increased oxygen surface diffusion at low temperature independently of the CGO effect. The barrier effect of the membranes and the induced solid/gas contact time were considered as key parameters to develop these porous nanophase ceramic membranes. (C) 2003 Published by Elsevier Science B.V.