The surface chemistry of barium strontium iron cobalt oxide of nominal composition Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-delta) has been studied using X-ray photoelectron spectroscopy (XPS). Precursor powders were prepared by a wet chemical process and sintered into pellets at 1100 degrees C. Polished pellets were examined by XPS at ambient and elevated temperatures, and after in situ heat treatments at temperatures up to 800 degrees C. Curve fitting and analysis of the spectral peaks was carried out, and significant changes in surface composition were observed as a result of the thermal treatments. At intermediate temperatures (300-500 degrees C) there was a reduction in barium and strontium concentrations relative to cobalt and iron. At 800 degrees C the surface concentration of barium and strontium increased. The nature of the oxygen species present on the surface also changed with temperature. Initially carbonate was dominant, but its concentration decreased with increasing temperature and a (Ba,Sr)-O species appeared. A significant amount of cobalt was reduced to 2+ at a temperature of 300 degrees C but reoxidised on cooling. (C) 2011 Elsevier B.V. All rights reserved.