As is true in solution, irradiation at 350 nm of 7-(10-thiodecoxy)coumarin (1) as a self-assembled monolayer on polycrystalline gold (Au-1) produces [2 + 2] photodimers. in the monolayer, photodimerization is indicated by differences in the grazing angle surface FTIR spectra, sessile drop contact angles, and surface fluorescence spectra before and after irradiation. The photodimerization can be reversed by irradiation at 254 nm, as demonstrated by changes in these same structural probes. Better photochemical reversibility and regioselectivity are observed when the in situ photodimerization is conducted as a dry solid monolayer than when in contact with benzene. A comparison of the structure of a self-assembled monolayer of a syn head-to-head (H-H) dimer of 7-(10-thiodecoxy)coumarin on polycrystalline gold (Au-2) with that of the photodimers generated by irradiation of Au-l demonstrates that surface reorganization is accomplished in the photodimerization-photoreversion cycle.