Kinetic demixing and decomposition were studied on three La0.6Sr0.4Co0.2Fe0.8O3-delta oxygen-separation hollow fibre membrane modules, which were operated under a 0.21/0.009 bar oxygen partial pressure difference at 950 degrees C for 1128, 3672 and 5512 h, respectively. The post-operation membranes were characterized by Secondary Ion Mass Spectrometry, Scanning Electronic Microscope, Energy Dispersive Spectrum and X-ray Diffraction. The occurrence of kinetic demixing and decomposition was confirmed through the microstructural evolution of the membranes. Secondary-phase grains were found on the air-side surface of the membranes after the long-term operation and Co and Fe enrichment as well as La depletion was found on the surface and in the bulk at the air side. Cation diffusivities were found to be in the order Co > Fe > Sr > La. Kinetic demixing and decomposition rates of the membranes at the air side were found to be self-accelerating with time: the role of A-site deficiency in the perovskite lattice in the bulk near the air side surface is implicated in the mechanism. The oxygen permeability was not affected by the kinetic demixing and decomposition of the material during long-term operation (up to 5512 h), however, we may expect permeability to be affected by secondary phase formation on the air-side surface at even longer operational times. (C) 2010 Elsevier B.V. All rights reserved.