X-ray photoelectron spectroscopy (XPS) was used to characterize the surfaces of mechanically treated wood. Microtomed and sanded surfaces of spruce, larch, beech and oak were investigated. Changes due to the sanding process were identified from the survey spectra as well as from the detailed C1s spectra. The changes were quantified with the atomic ratio of oxygen to carbon and with a detailed analysis of the contributions to the C1s peak. The identified changes were explained in terms of the macromolecular wood substances and in terms of density and the amount of extractives.