The interaction of an X-ray-modified self-assembled monolayer with a mixture of atomic and molecular oxygen (O/O-2) has been studied using in situ X-ray photoelectron spectroscopy. Initially the reaction dynamics are dominated by the incorporation of new oxygen containing functionality at the vacuum/film interface. At intermediate O/O-2 exposures, when a steady-state concentration of C-O, C=O, and O-C=O groups has been established, the production of volatile carbon-containing species, including CO2, is responsible for etching the hydrocarbon film. Upon prolonged O/O-2 exposures, O atoms penetrate to the film/substrate interface, producing Au2O3 and sulfonate (RSO3) species. Under steady-state conditions, the thickness of the hydrocarbon film was reduced with an efficiency of approximate to7.4 x 10(-4) Angstrom/impingent O atom while the average penetration depth of O atoms within the hydrocarbon film was determined to be approximate to5.5 Angstrom