The time-dependent degradation of anode-supported Solid Oxide Fuel Cells (SOFCs) with La0.58Sr0.4Co0.2Fe0.8O3-delta (LSCF) cathodes has been studied. Eight SOFCs have been tested over a period of 1000 It under different operation conditions to investigate the influence of different operation parameters on the degradation of the electrochemical performance. The cells were tested at 700 or 800 degrees C, at 0.3 or 0.6 A/cm(2) and with 21% or 5% O-2 at the cathode side and showed performance losses of 2-4% per 1000 h. While an elevated temperature and an elevated oxygen partial pressure had a negative influence on long-term performance, the current density did not have a clear effect. Material analysis of the cells showed a formation of SrZrO3 at the interface of the Ce0.8Gd0.2O2-delta interlayer and the yttria stabilized zirconia (8YSZ) electrolyte during sintering of the cathode. There are indications of a further formation of this phase during the electrochemical characterization obtained from X-ray diffraction analysis on LSCF-YSZ powder mixtures that were exposed to 800 degrees C for 200 h. (c) 2006 Elsevier B.V. All rights reserved.