The hydrogenation of 1,3-hexadiene over a Pd/silica catalyst has been investigated using in situ FTIR spectroscopy. This study shows that the presence of an organic species on the surface of the catalyst can have a marked effect on the rate of hydrogenation of 1,3-hexadiene. Toluene-d(8) was coadsorbed on the surface of the catalyst and, depending on the order of introduction of the reactants to the system, has been shown to act as either a promoter or a poison for the hydrogenation of the diene. Introduction of the toluene-d(8) prior to the diene has a promoting effect with the toluene-d(8) acting as a H-transfer agent. When the toluene-d(8) is introduced after the diene it acts as a poison, blocking active sites and covering the surface. The ability of toluene-d(8) to act as a promoter or poison appears to be dependent on the availability of active sites at the time of its introduction to the reaction system. To act as a promoter, or H-transfer agent, the toluene-d(8) must first be able to form a bonds to the surface by dissociation of the D atoms of the methyl group. When the site availability is poor, the toluene-d(8) retains its molecular identity upon adsorption, thus blocking active sites and reducing the rate of hydrogenation.