Surface-enhanced Raman spectra were obtained from p-nitrobenzoic acid (PNBA) adsorbed onto silver island films that had previously been coated with self-assembled monolayers of alkyl mercaptans. PNBA adsorbed onto silver as a p-nitrobenzoate ion, penetrating through the hydrocarbon layer. However, the alkyl coating hindered the chemisorption and revealed the existence and location of two types of adsorption sites. The photo-induced reduction of PNBA(1,2) was not observable when the molecule was adsorbed on silver previously coated with hexadecyl mercaptan. In general, the reaction was much slower when silver was coated with hydrophobic layers than when it was bare or coated with hydrophilic layers. These results are discussed and interpreted in favor of a polymolecular mechanism for the photoinduced reaction.