Quantitative determination of average rhodium oxidation state for supported rhodium catalyst was examined by using Rh L-3-edge XANES (X-ray absorption near-edge structure) spectra. A series of Rh-loaded SiO2 catalysts (Rh2Ox/SiO2) containing different amount of Rh metal and Rh2O3 phases were prepared, and the average oxidation number (x) was determined by the amount of CO2 formed during CO reduction treatment as the final step of catalyst preparation. L-3-edge XANES spectra of these samples were recorded, and the white line area intensity of the spectra was evaluated. A good linear relationship was confirmed between the white line area intensity and the average oxidation number (x), indicating that the oxidation state of Rh species in structurally unknown Rh samples could be quantitatively determined by a simple L-3-edge XANES analysis. The effect of the oxidation number (x) on the Rh2O/SiO2-catalysed CO oxidation was discussed, in order to demonstrate a utility of this method in a catalytic study. (C) 2011 Elsevier B.V. All rights reserved.