In situ FT-IR spectroscopy was used to study the surface species involved in the selective epoxidation of propene with an O-2/H-2 mixture over a deactivating 1 wt% Au/TiO2 catalyst and a non-deactivating 1 wt% Au/TiO2/SiO2 catalyst. Propene adsorbs weakly on both catalysts via gamma -hydrogen bonding with surface hydroxyl groups of the TiO2 or TiO2/SiO2 support. Propene adsorption is completely reversible at temperatures between 300 and 400 R. Irreversible adsorption of propene oxide (PO) was observed on both catalysts, yielding bidentate propoxy moieties. Similar propoxy species are formed after prolonged exposure of the catalysts to a propene/oxygen/hydrogen mixture. Deactivation of TiO2 catalysts is explained by the formation of these propoxy groups on Ti sites active and selective in propene epoxidation. Neighboring acidic Ti sites are involved in coupling of PO onto these sites. Occupation of selective sites with propoxy groups is limited on TiO2/SiO2 catalysts. On this support, propoxy groups are located on acidic agglomerated TiOx sites and linked to an Si-OH functionality. These groups are not involved in the selective epoxidation of propene, which occurs over isolated tetrahedral sites. Besides propoxy groups, surface compounds with absorptions in the C=O stretching range (acetone and propanal) were observed on both catalysts. Formate and acetate species (implying C-C bond cleavage) were identified exclusively on the Au/TiO2 catalyst. The relevance of these species to the selectivity of the catalysts investigated is discussed.