Material stability and cation transport of La 0.6Sr 0.4Co 0.2Fe 0.8O 3-δ were investigated by long-term annealing of the sintered pellets with and without applying oxygen potential gradients. The heat treatments at 1073 - 1273 K under uniform p(O 2) of 1.0 × 10 -4 - 0.21 bar for one week did not make any notable changes on the pellets. However, significant changes were observed when the pellets were placed under oxygen potential gradient between 1.0 bar oxygen and diluted oxygen (1.0 × 10 -4 - 1.0 × 10 -1 bar of p(O 2)). On the high p(O 2) side, many particles of cobalt and strontium oxide were found on the surface at 1273 K. The amount of the deposited particles increased with decreasing oxygen potential on the opposite side and/or increasing temperature. The results indicated that the cobalt and strontium ions are faster than the other cations and the cobalt ions are fastest among the cations involved in the LSCF.