During the long-term operation of a La0.58Sr0.4Co0.2Fe0.8O3-delta (LSCF) membrane at 800 C for nearly 3000 h, a stable permeation flux of (0.15 +/- 0.01) ml cm(-2) min(-1) oxygen was achieved. Nevertheless degradation of the membrane surfaces due to kinetic demixing of mobile cations was observed. Different oxide layers were found on the feed and the permeate surfaces of the membrane. (C) 2010 Elsevier B.V. All rights reserved.