Argon plasma treatment and subsequent atmospheric exposure have been used to incorporate new oxygen functionalities at the surface of polystyrene (PS) and polypropylene (PP). High-energy resolution X-ray photoelectron spectroscopy (XPS) has yielded molecular information regarding the site of modification in both polymers. Core level and valence band spectra have been interpreted. We have adopted a simple subtraction process to highlight the changes occurring in the valence band spectra upon treatment. In the case of PS, modification is found to be occurring at ring sites. In PP, modification is thought to be occurring at two sites. We propose that modification at the tertiary carbon site leads to chain scission and subsequent etching, while modification at the methyl side group results in functionalities being incorporated at the polymer surface. These data help to substantiate mechanisms that were previously proposed, based on the accepted mechanisms of cross-linking vs chain scission for these polymers.