The performance stability and degradation mechanism of La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF) cathodes and LSCF impregnated Gd0.1Ce0.9O2-delta (LSCF-GDC) cathodes are investigated under solid oxide fuel cell operation conditions. LSCF and LSCF-GDC cathodes show initially performance improvement but degrade under cathodic polarization treatment at 750 degrees C for 120 h. The results confirm the grain growth and agglomeration of LSCF and in particular GDC-LSCF cathodes as well as the formation of SrCoOx particles on the surface of LSCF under cathodic polarization conditions. The direct observation of SrCoOx formation has been made possible on the surface of dense LSCF electrode plate on GDC electrolyte. The formation of SrCoOx is most likely due to the interaction between the segregated Sr and Co from LSCF lattice under polarization conditions. The formation of SrCoOx would contribute to the deterioration of the electrocatalytic activity of the LSCF-based electrodes for the O-2 reduction in addition to the agglomeration and microstructure coarsening. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.