When hexadecylsiloxane monolayers are immersed in solutions of bromine in carbon tetrachloride and irradiated with a tungsten lamp, C-H bonds near the surface of the monolayer undergo free-radical bromination. The bromine introduced into the monolayers can easily be substituted by small nucleophiles. prepared from 16-bromohexadecyltrichlorosilane, but with minor differences arising due to different amounts and distributions of bromine within the two types of monolayers.