The chemical state of oxygen on the surfaces of mixed electronic-ionic conducting La0.6Sr0.4Co1-yFeyO3 ceramics was characterized by X-ray photoelectron spectroscopy (XPS). It was ascertained that there are five different kinds of oxygen on the ceramic surfaces, including lattice oxygen (O-L), chemisorbed oxygen (O-C) in the forms of O2-, O-, and O-2(-), and oxygen in hydroxyl environment (O-H). The concentration of O-C + O-H relative to total detected oxygen enhanced with the increase of Co/Fe ratio. In order to examine the relation between the chemical states of oxygen on the surfaces and the electrical nature of the ceramics, the mixed electronic-ionic conducting properties were investigated. At an identical measuring temperature, the electronic conductivity and ionic conductivity of La0.6Sr0.4Co1-yFeyO3 ceramics tended to rise with the increase of Co/Fe ratio. It was considered that the mixed electronic-ionic conducting properties Lire responsible for the complex chemical states of oxygen on the ceramic surfaces. (C) 2004 Elsevier B.V. All rights reserved.