UV-initiated (254 nm) hydrosilylation of hexadecene on Si(111)-H has been studied in the presence of various aliphatic and aromatic molecules (additives). Many of these additives cause an enhancement in the pseudo-first-order rate coefficient (k(obs)) of hydrosilylation, some up to 200X faster than observed in neat hexadecene. It is proposed that these additives capture the photoejected electron from the surface, thereby increasing the probability of reaction of the alkene with the surface hole (h(+)), leading to Si-C bond formation. While the ability of these additives to increase k(obs) is related to their reduction potential, aromatic additives are particularly efficient; we suspect this is due to the relatively strong physisorption of the aromatic molecules leading to a favorable geometry for electron transfer. The presence of these additives permits the use of a much lower intensity of UV light (similar to 30 mu W/cm(2)), reducing the probability of photodegradation of the monolayer, and maximum coverage can be reached within minutes.