X-ray photoelectron spectroscopy (XPS) has been employed in an investigation of the structure and photooxidation of self-assembled monolayers (SAMs) formed by the immersion of evaporated Au films into ethanolic solutions of 1-octanethiol and 1,8-octanedithiol. XPS has been used to confirm unambiguously that individual molecules in SAMs formed from 1,8-octanedithiol and 1-octanethiol are in both cases attached to the surface through a single Au-thiolate bond. In the case of the dithiol this is consistent with an "upright" alignment of the hydrocarbon chains (perpendicular to the surface) rather than a "looped" configuration dictated by the simultaneous binding of both ends of the molecule to the surface. Photooxidation at the SAM/Au interface was not detected after 3 h of exposure of either SAM to laboratory lighting and air, indicating that the monolayers provided an effective barrier against the penetration of atmospheric oxygen to the substrate. In the case of 1,8-octanedithiol, however, photooxidation occurred at the omega-thiol group (RSH), remote from the surface and exposed at the SAM/air interface, to yield a sulfonic acid species (RSO3H). It is proposed that the most likely mechanism for this reaction involves the transfer of "hot" (subvacuum) electrons from the Au surface to the omega-thiol group at the SAM/air interface followed by reaction with proximal atmospheric oxygen. In cases where atmospheric oxygen can penetrate to the Au surface, a similar mechanism is proposed to explain the photooxidation of thiolate (RS-Au) to sulfonate (RSO3-Au), which is commonly observed in alkanethiol SAMs.