The interaction of vapor-deposited Al atoms with self-assembled monolayers (SAMs) of HS(CH2)(15)CH3 and HS(CH2)(15)CO2CH3 chemisorbed at Au{111} surfaces was studied using X-ray photoelectron spectroscopy, infrared spectroscopy, time-of-flight secondary ion mass spectrometry, and spectroscopic ellipsometry. For the CH3-terminated SAM, no reaction with C-H or C-C bonds was observed. For total Al doses up to similar to 12 atoms/nm(2), penetration to the Au-S interface occurs with no disruption of the average chain conformation and tilt, indicating formation of a highly uniform similar to 1:1 Al adlayer on the Au. Subsequently, penetration ceases and a metallic overlayer begins to form at the SAM-vacuum interface. These results are explained in terms of an initial dynamic hopping of the -S headgroups on the An lattice, which opens transient diffusion channels to the Au-S interface, and the closing of these channels upon completion of the adlayer. In contrast, Al atom interactions with the CO2CH3-terminated SAM are restricted to the vacuum interface, where in the initial stages discrete organometallic products form via reaction with the CO2CH3 group. First, a I:1 complex forms with a reduced C=O bond and an intact CH3 moiety. Further exposure leads to the additional reaction of about four Al atoms per ester, after which a metallic overlayer nucleates in the form of clusters. After the growth progresses to similar to 30 Angstrom, the clusters coalesce into a uniform metallic film. These results illustrate the extraordinary degree of control that organic substrates can exert during the course of metal film formation.