The stability and surface composition of La0.6Sr0.4Co0.2Fe0.8O3 - delta (LSCF)-Sm0.2Ce0.8O2 (SDC) cathodes prepared by impregnation are investigated at 750 degrees C under open circuit and different oxygen partial pressures. The performance degradation during the test at Po-2 = 0.21 atm is closely related to the low frequency mass transfer process. Little change of the electrochemical performance of cathodes is observed after 24 h test at Po-2 = 0.001 atm. XPS patterns show that the decrease in oxygen partial pressure leads to the suppression of Sr enrichment on the surface of cathodes and retards the performance degradation. (C) 2015 Elsevier B.V. All rights reserved.