Two reactions, the hydrogenation of buta-1,3-diene on a Pd8Ni92(I 1 1) alloy and the hydrogenation of benzene on Pt(1 1 1), have been investigated under catalytic conditions by reflection absorption infrared spectroscopy (RAIRS). The purpose of this study was to use infrared spectroscopy to observe both the consumption/production of the different reactants/products and the adsorbed species present during the reaction. On Pd8Ni92(1 1 1), only butenes were produced during the buta-1,3-diene hydrogenation reaction up to the quasi-complete conversion of the diene, with a high rate of reaction. This alloy is therefore, active and very selective for such a reaction. In this case, no absorption bands corresponding to surface species were observed. Ad-species were observed during the benzene hydrogenation reaction on Pt(I 1 1); they cannot be attributed to chemisorbed molecular benzene, but are more probably due to partially hydrogenated intermediates, such as cyclohexadiene and/or cyclohexene.