The adsorption and subsequent reactions of 2,5-dimercapto-1,3,4-thiadiazole (DMcT) and 2-mercapto-5-methyl-1,3,4-thiadiazole (McMT) on polycrystalline gold films have been characterized by X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, IR spectroscopy, mass spectrometry, and scanning tunneling microscopy. Adsorption from solution was studied as well as vapor deposition under ultrahigh vacuum conditions. McMT adsorbs from an ethanolic solution to Au surfaces but decomposes to a large extent to give nonstoichiometric, nitrogen-deficient thin films containing smaller fragments, whereas DMcT forms thin films on Au surfaces consisting of intact molecules and oligomers resulting from surface polymerization. A temperature-dependent XPS study shows that McMT forms a thin film of intact molecules after deposition from the vapor phase at a substrate temperature of 130 K, which decomposes above 300 K to yield new, stable surface species.