The oxygen exchange kinetics of La0.6Sr0.4CoO3-delta was investigated at 700 degrees C and p(O-2) = 0.10 bar by in-situ conductivity relaxation measurements. The experiments were performed for 1000 h in an O-2-Ar atmosphere with and without 50 ppm SO2. For post-test analyses, scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDXS) and analytic transmission electron microscopy (TEM) were applied. Elemental depth profiles were obtained by X-ray photoelectron spectroscopy (XPS). The decrease in the chemical oxygen exchange coefficient k(chem) by a factor of 10 during the course of 1000 h indicated that an accumulation of traces of sulphur (estimated to the range of ppb) in the form of SrSO4 can lead to a significant degradation even in nominally pure O-2-Ar. The exposure to 50 ppm of SO2 for 1000 h caused a severe degradation in k(chem) by a factor of 3000. Post-test analyses showed that the perovskite phase La0.6Sr0.4CoO3-delta is decomposed into SrSO4 and La2O2SO4 within the first 500 nm of the surface, and a nano-crystalline Co-rich phase, which is located in the depth range of 500-1400 nm. (C) 2013 Elsevier B.V. All rights reserved.