Conversion of acetaldehyde to crotonaldehyde on anatase TiO2 films was studied by in situ Fourier transform infrared (FTIR) spectroscopy and by density functional theory (DFT) calculations. In situ FTIR showed that acetaldehyde adsorption is accompanied by the appearance of a hitherto non-assigned absorption band at 1643 cm(-1), which is shown to be due to acetaldehyde dimers. The results were supported by DFT calculations. Vibrational frequencies calculated within a partially relaxed cluster model for molecular acetaldehyde and its dimer, and for the corresponding adsorbed species on the anatase (101) surface, were in good agreement with experimental results. A kinetic model was constructed based on the combined FTIR and OFT results, and was shown to explain the essential features of the acetaldehyde condensation reaction. (C) 2013 Elsevier B.V. All rights reserved.