Three perovskite-type compounds of composition La0.4Ca0.6Fe1-xCoxO3-delta (x = 0, 0.25 and 0.5) were investigated for use as oxygen separation membranes for the partial oxidation (POX) of methane to syngas. Special attention was given to the question of their stability in real operating conditions. A permeation set-up was specially designed to measure oxygen fluxes through these materials when placed in a strong pO(2) gradient. It also facilitated testing the long-term stability of the specimen. Permeation measurements performed in an air/argon gradient between 800 and 1000 degreesC showed that the highest fluxes were obtained with the highest content of cobalt (La0.4Ca0.6Fe0.5Co0.5O3-delta congruent to La0.4Ca0.6Fe0.75Co0.25O3-delta > La0.4Ca0.6Fe0.75). In addition, comparison between the fluxes of samples of different thickness gave clear evidence of surface limitations in the oxygen transport. The long-term stability test showed opposite trends: only the two lowest Co containing compounds (x = 0 and 0.25) sustained an air/(Ar + H-2) gradient over more than 600 h. The other (x = 0.5) broke shortly after the introduction of H-2. In the presence of H-2, the oxygen flux was increased by a factor 10 compared to Ar and reached 0.83 mumol/cm(2) s for La0.4Ca0.6Fe0.75Co0.25O3-delta at 900 degreesC. Post-operation SEM examination of the cross-section and both surfaces revealed that the surface exposed to H-2 had started to decompose resulting in the formation of a thin porous layer but the bulk of the material remained unchanged. (C) 2003 Elsevier Science B.V. All rights reserved.