Self-assembled monolayers of alkyl mercaptans were used as spacer layers on silver island films. N-Alkylmercaptans (8, 12, and 16C) yielded low-energy, hydrophobic upper surfaces but carboxyl-terminated undecyl mercaptan gave a hydrophilic upper surface. Surface-enhanced Raman scattering (SERS) was observed from two different molecules deposited on the alkyl mercaptans/island film system. One was p-nitrobenzoic acid (PNBA), a small molecule able to chemisorb on silver. The other, cobalt phthalocyanine (CoPC), was bigger and only physisorbed on the substrates. For every spacer/scatterer pair, special care was taken of the exact location of the molecule. The small molecule (PNBA) allowed characterization of the coverage of both types of spacer layers, hydrophobic and hydrophilic. PNBA could always penetrate into the alkyl chains, whatever the spacer. Substrates with hydrophobic and hydrophilic upper surfaces gave very different spectra after deposition of the bigger molecule (CoPC). When CoPC was located at about 15-20 Angstrom from silver, the SERS signal was about 10 times lower than when CoPC was adsorbed directly onto the metal. These results constitute a first measurement of the spatial extension of SERS with strong evidence that the scattering molecule was not on the metallic surface.