Surface species formed during the adsorption of acetaldchyde at 300-673 K on TiO2-supported Pt, Rh and Au catalysts were investigated by Fourier transform infrared spectroscopy. Two forms of molecularly adsorbed acetaldehyde-H-bridge bonded on surface OH groups and adsorbed on Lewis sites through one of the oxygen lone pairs-were identified. beta-aldolization of acetaldehyde led to the formation of crotonaldehyde, which adsorbs on Lewis sites of TiO2 through one of the oxygen lone pairs and on metallic sites via the C atom of the aldehyde group. Adsorbed acetaldehyde can be oxidized into surface acetate and it can be reduced resulting in adsorbed ethoxy. Mass spectroscopic analysis of the gas phase composition revealed that the formation of gas phase products (crotonaldehyde, water, benzene, hydrogen, ethylene, acetylene and methane) depends on the nature of the metals and the reaction temperature. An attempt was made to find a possible link between the surface species and the formation of the primary gas phase products. (c) 2005 Elsevier B.V. All rights reserved.