The long-term degradation of the surface oxygen exchange kinetics of the IT-SOFC cathode material La0.58Sr0.4Co0.2Fe0.8O3-delta (LSCF) is investigated in dry and wet O-2-Ar atmospheres at 600-700 degrees C. Conductivity relaxation measurements indicate high chemical surface exchange coefficients (k(chem)) in dry O-2-Ar atmosphere at 600-800 degrees C with E-a(k(chem))similar to 200 kJ mol(-1). However. during 1 kh an exponential decrease of k(chem) is observed at 600 degrees C in dry atmosphere. After switching to a H2O-containing atmosphere a further step-wise decrease of k(chem) by a factor of 2 occurs, followed by an approximately linear long-term degradation with a rate of similar to 10% kh(-1). At 700 degrees C a pronounced decrease of the surface exchange coefficient by 2 orders of magnitude is found after switching from dry to wet atmospheres. Post-test XPS analysis indicates that the deterioration of the catalytic activity of LSCF originates in significant changes of the surface cation composition (especially Sr enrichment) and silicon poisoning. (C) 2010 Elsevier B.V. All rights reserved.