Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to examine the effect of thermal treatment of self-assembled monolayers (SAMs) produced from various thiols (1-hexanethiol, 1-decanethiol, 1-octadecanethiol, 1,6-hexanedithiol, 1,10-decanedithiol, 1,2-benzenedithiol) on a 5 wt % Pt/Al2O3 catalyst. Catalysts were characterized during heating and cooling between 290 and 553 K in both the presence and absence of H-2. Overall, the behavior of thiols on Pt/Al2O3 in an inert He environment was similar to the behavior reported in other works on Au, although in the case of the Pt catalyst, C-H bond dissociation in the thiols was apparent at high temperatures. Under H-2 flow, however, markedly different behavior was observed; in particular, conformational order was observed to increase with increasing temperature, up to temperatures as high as 500 K for octadecanethiol-coated catalysts. The effects of H-2 exposure are much less pronounced for alkanedithiol-coated catalysts. 1,2-Benzenedithiol was found to undergo partial hydrogenation under H-2, indicating that hydrogenating reaction conditions can also influence the chemical structure of the monolayer on active metals, such as Pt. The differences in thiolate structure caused by high-temperature exposure to hydrogen were found to have a significant effect on the rate and selectivity for hydrogenation of prenal, indicating that such effects may be broadly important in the use of thiolate-promoted catalysts for hydrogenation reactions.