The X-ray induced modification/defluorination of semifluorinated self-assembled monolayers (SAMs), based on 1H,1H,2H,2H-perfluorodecanethiol (CF3(CF2)(7)(CH2)(2)SH) has been studied using X-ray photoelectron spectroscopy (XPS). At short irradiation times X-ray induced defluorination of the semifluorinated SAM exhibits first-order kinetics with respect to the film's fluorine concentration. The evolution of the C(1s) region during modification is consistent with a kinetic model of defluorination involving consecutive C-F bond breaking events (e.g., CF2 --> CF). Relative defluorination rate constants obtained for the different fluorine-containing functionalities (e.g., CF2, CF) provide support for a stochastic fluorine loss process, where each individual C-F bond is initially equally labile toward dissociative electron attachment, independent of local chemical environment. Upon atmospheric exposure the modified film's oxygen adsorption characteristics demonstrate that the density of radicals produced within the organic film during X-ray exposure exhibit a non-linear dependence on irradiation time due to the onset of radical-radical coupling reactions at longer X-ray exposures. The appearance of oxygen within the film is also responsible for further defluorination due to the reactivity of peroxy radicals (-CF(OO .)-). Results from this study support the idea that SAMs can be employed as model systems for developing a detailed understanding of the molecular level events associated with organic surface modification processes.