X-ray photoelectron spectroscopy (XPS) was used to investigate the valence state of cerium in unsupported composite CexMn1-xO2-y catalysts. Manganese-cerium composite oxides are being widely used in sub- and supercritical catalytic wet oxidation for the treatment of wastewater containing toxic organic pollutants. The performance of such catalysts depends, among others, on the redox reactions involving CeO2-x suboxides and manganese oxides. In order to investigate the surface chemistry of such environmental catalysts, 10 samples with Mn/Ce ratios ranging from 0/10 to 9/1-molar basis-were synthesized by co-precipitation and characterized as-prepared and after gold coating. Principal component analysis and curve fitting of Ce 3d and O 1s core-level spectra were used to determine the types of valence states and their proportions on the catalysts' surface. The study revealed valence state modifications in cerium and changes in the amount of lattice oxygen depending on the Mn/Ce ratio and on whether or not the as-prepared catalysts were subjected to gold deposition. The proportion of surface Ce3+ species in the non-stoichiometric CeO2-x oxides was successfully quantified through linear correlation between nu(2)' peak area and integral of the Ce4+ 3d spectrum. (C) 2002 Published by Elsevier Science B.V.