We have used infrared-visible sum-frequency generation (SFG) to probe spectroscopically the interfacial structure of a self-assembled monolayer of allkyl thiol on a Au(111) surface. The SFG spectra of the CH3 group, measured as a function of azimuthal angle, is not consistent with sulfur atoms being situated at sites of a single type, e.g. hollow or bridge, but is consistent with a mixed arrangement.