Methanethiol adsorption onto Au(111) was studied in situ by scanning tunneling microscopy in air at room temperature. By maintaining registry on the surface during adsorption, it was demonstrated that gold vacancy islands (i.e., "etch pits"), which are created by the chemisorption of methanethiol, exist in rows that exactly follow the (22 x root 3) gold reconstruction. Ostwald ripening produces larger gold vacancy islands that are no longer coincident with the original Au(111) reconstruction. Further annealing produces well-ordered domains of methanethiol that have the same surface structures observed for other alkanethiol-based self-assembled monolayers on Au(111). The results demonstrate for the first time that at high rates of dosing, the structure of a fully-formed thiol monolayer on Au(111) is influenced by the original (22 x root 3) reconstruction of the underlying gold surface. These data strongly suggest that the gold vacancy islands observed in methanethiol, thiophene, and other alkanethiol-based monolayers are not the result of chemical etching.