Oxygen flux through La0.5Sr0.5Fe1-xCoxO3-delta (x = 0, 0.5 and 1) membranes has been determined as a function of oxygen partial pressure, temperature and time. The flux was diffusion controlled for low pO(2) gradients while larger pO(2) gradients caused a surface exchange controlled flux. The activation energy of the oxygen flux varied in the range 67-105 kJ/mol. After about 1 month at 1150 degrees C in an O-2/N-2 gradient the membranes were examined for kinetic demixing and decomposition. On the reducing side only the original perovskite phase was observed at the surface, while on the oxidizing side various secondary phases were observed dependent on the composition at the Fe/Co-site and the Sr+La/Fe +Co ratio of the materials. Moreover, kinetic demixing of the main perovskite phase was also observed, particularly near the surfaces. Grain growth and pore coalescence resulting in membrane expansion were also observed in some cases. The present findings are discussed with regard to the long term chemical stability of the membranes. (c) 2006 Elsevier B.V. All rights reserved.