Commercially available poly[methyl(mercaptopropyl)siloxane] oligomers have been chemisorbed to gold surfaces from organic solution via gold-alkanes thiolate bonds. Bound films of monolayer thickness (15-20 angstrom) are shown to form spontaneously and are compositionally and structurally isotropic, regardless of solution concentration. X-ray photoelectron spectroscopy yields evidence for two distinct sulfur species in these films : one consistent with gold-thiolate bonds and one representing free, unbonded thiol. A high content of unbonded thiol has been used to modify these organic films in situ using photochemistry. Subsequent exposure of the assemblies to monomeric organothiols does not change film properties. We demonstrate that these siloxane oligomers can be photochemically modified in bulk and assembled on surfaces or, alternatively, that the siloxanes can be chemisorbed as films and subsequently modified in situ to present various functional surfaces. Photochemical derivatizations included alkyl, perfluoor, and polyether chains. Cyclic voltammetry exhibited various redox blocking capabilities for Fe(CN)63- probes depending on film preparation conditions and chemistry. Chemical derivatization disrupted film structure and redox blocking ability, in addition to oxidizing unbound thiol groups. Both pure and photoderivatized siloxane films were perturbed by high-temperature (>100-degrees-C) exposure to solution environments.