Self-assembled monolayers (SAMs) of methyl- and vinyl-terminated n-alkanethiolates on gold have been exposed at 305 K to beams of atomic fluorine in an ultrahigh-vacuum environment. In situ X-ray photoelectron spectroscopy of the resulting monolayers indicates that the incoming fluorine atoms penetrate the close-packed chains and react to form mono- and difluorinated methylene groups without fluorination of the gold substrate and with little or no loss of carbon atoms. No reaction was observed with molecular fluorine. After a cumulative dose of similar to 680 fluorine atoms per carbon chain, approximately seven of the 20 chain carbon atoms have been fluorinated. The fluorination most likely occurs by a mechanism in which an incoming fluorine atom abstracts a chain methylene hydrogen atom, creating a radical site for subsequent addition of a fluorine atom. Kinetic modeling of the fluorine atom uptake indicates that the cross sections for hydrogen abstraction and fluorine addition are somewhat smaller than their gas phase counterparts, suggesting that steric crowding limits the overall fluorine atom uptake. No significant differences between the reactivities of the two differently terminated SAMs could be discerned. Our results indicate that C20-alkanethiolate SAMs are effective coatings for preventing oxidation of metallic surfaces by gas phase reactants.