The interaction of vapor-deposited Al atoms with self-assembled monolayers of HS(CH2)(15)CO2H chemisorbed at polycrystalline Au(111) surfaces has been studied using time-of-flight secondary-ion mass spectrometry, X-ray photoelectron spectroscopy, and infrared spectroscopy. The Al deposition was performed incrementally at room temperature. The Al atoms do not penetrate into the organic monolayer, but rather they remain at the vacuum interface where they undergo chemical interactions solely with the CO2H groups. Reaction of the CO2H groups continues until slightly more than one atom per reacting group is deposited, on average; thereafter, no further reaction is observed. However, 20-25% of the CO2H groups remain unreacted, regardless of the Al coverage. These results are explained on the basis of a combination of chemical and steric effects.