We report the gas-phase photochlorination of methyl-terminated alkyl monolayers on silicon. We argue that this process proceeds by a free-radical mechanism, as is the case for homogeneous photochlorination. This is an example of what should be a family of convenient methods for the incorporation of various functional groups into simple alkyl monolayers by chlorine-radical activation. Monolayers prepared from 1-octadecene on Si(111) were exposed to Cl-2 with illumination at 350 nm. We observe that a fraction of the carbon atoms on the surface become singly chlorinated and a smaller fraction become doubly chlorinated, as measured by the chemically shifted components of the C 1s X-ray photoelectron spectrum. The elemental composition of the resulting monolayers, film thickness, and contact angles are reported as a function of exposure. Under conditions that chlorinate only a fraction of the carbon atoms, IR spectroscopy reveals complete chlorination of the methyl groups, demonstrating a strong steric preference for chlorination at the ends of the alkyl chains. In the absence of illumination,,a much slower dark reaction is observed.