Sulfur poisoning behavior of LSCF cathodes with different Sr contents, (La0.6Sr0.4)(Co0.2Fe0.8)O-3. LSCF6428 and (La0.8Sr0.2)(Co0.2Fe0.8)O-3, LSCF8228, was investigated at T = 800 degrees C in air with 1 ppm SO2. The sulfur distribution of the cathodes was derived from the SEM/EDX mapping and was identified due to the SrSO4 formation. Although this SrSO4 formation can be recognized as the oxidative reaction, this took place mainly in the vicinity of the electrolyte/cathode interfaces where the electrochemical reaction occurs and therefore the oxygen potential is lowered due to the over-potential. Moreover, the width of the SrSO4 formation was wider in LSCF6428 than in LSCF8228. These features suggest that the SrSO4 formation is governed mainly by some kinetic factors in the electrochemical oxygen reduction; roles of oxide ion vacancies, the presence of dissociated oxygen, and the thermodynamic activity of SrO are discussed in relation to the SrSO4 formation. (C) 2012 Elsevier B.V. All rights reserved.