The chemical states of the surface of (Ln(0.5)Sr(0.5))CoO3-delta (Ln (lanthanides) = Pr, Nd and Sm) used for cathode materials of intermediate temperature operating solid oxide fuel cells (IT-SOFCs) were investigated by X-ray photoelectron spectroscopy (XPS). Oxygen peaks comprised of lower binding energy (LBE) and higher binding energy (HBE) peaks from (Ln(0.5)Sr(0.5))CoO3-delta and Pr0.3Sr0.7CoO3-delta (PSC37) showed that some merged oxygen peak behavior is a function of the Sr and lanthanide concentrations. By investigating the oxygen peaks, it was determined that more oxygen vacancies were generated on the surface of the cathodes when the lanthanides and Sr were substituted into perovskite oxides. When comparing the binding energies (BEs) of PSC37 with Pr0.5Sr0.5CoO3-delta (PSC55), the LBE and HBE of the Sr peaks both increased when Sr was substituted at the A-site of a perovskite. Surface analysis of the Co peak on the surface of the cathode materials showed that the Co exists mainly as Co3+ and partially oxidized to Co4+ on the cathode materials. The partial existence of Co4+ can provide some polaron hopping providing electronic conduction for the solid oxide fuel cell. (C) 2011 Elsevier B. V. All rights reserved.