The adsorption of acetaldehyde on particulate TiO2 surfaces has been studied at 233 k by FT-IR spectroscopy using a specially designed IR cell. It has been found that acetaldehyde initially adsorbs onto the surface through hydrogen bonding and Lewis acid sites. As the temperature is raised to 251 K, the spectroscopic evidence of formation of 3-hydroxybutanal surface intermediate is observed during aldol condensation reaction. The presence of this transient species has been characterized by the infrared features at 3185 cm(-1)-nu(OH), 1334 cm(-1)-delta(CH), 1273 cm(-1)-delta(COH), and 1105 cm(-1)-nu(C-C) and delta(COH). The assignments of all surface species are confirmed by adsorbing pure 3-hydroxybutanal and 2-butenal on TiO2 surface. A reaction mechanism consistent with the detectable surface intermediates is proposed. (C) 2008 Elsevier Inc. All rights reserved.