The stability of La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF6428) was studied by long-term annealing of the sintered pellets with and without oxygen potential gradients. The heat treatments at 1073-1273 K under uniform oxygen partial pressure of 10(-4) - 0.21 bar for one week did not make any notable changes in the composition of the pellets. However, significant changes were observed in the morphology and the composition at the surface of the high p(O-2) side (1 bar oxygen side) when the pellet was placed under oxygen potential gradient between 1 bar oxygen and diluted oxygen from 10(-4) to 10(-1) bar. On the high p(O-2) side, many particles containing cobalt and strontium, possibly cobalt oxides and strontium oxides, were found along the grain boundary by the scanning electron microscopic observation and the energy dispersive X-ray analysis. The amount as well as size of the deposited particles increased with decreasing oxygen potential on the low oxygen partial pressure side, i.e. increasing the oxygen potential gradient, and/or with increasing temperature. The results indicated that kinetic decomposition took place since the diffusions of cobalt and strontium ions are faster than those of the other cations. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.063210jes] All rights reserved.