The adsorption of propene on supported gold nanoparticles has been experimentally identified as a reaction step in the hydro-epoxidation of propene. This new finding was made possible by applying a detailed analysis of in situ measured XANES spectra. For this purpose, gold-on-silica catalysts were investigated since this support is more inert and propene is not converted. Propene adsorption was investigated by using the hydrogen oxidation as probe reaction. It was shown that co-feeding of propene dramatically decreased the hydrogen oxidation rate. Since it has been reported in the literature that the hydrogen oxidation occurs exclusively over gold nanoparticles. this inhibition by propene can be attributed to adsorption of propene on the gold nanoparticles. Delta-mu analysis of the in situ XANES spectra confirmed the adsorption of propene on the gold and the mode of adsorption was determined to be pi-bonding. Comparative experiments with ethene and propane confirmed this pi-bonded adsorption, since ethene similarly inhibited the hydrogen oxidation, while propane had only a minor effect. The direct observation of the adsorption of propene on gold nanoparticles corroborates our recent findings, in which we have shown that gold nanoparticles were activating propene to reactively adsorb on titania producing a bidentate propoxy species. (c) 2008 Elsevier Inc. All rights reserved.