Adsorption and further surface transformations of acetaldehyde on Al2O3-supported noble metal catalysts were studied by FTIR spectroscopy. Two forms (H-bridge bonded and bonded to Lewis sites) of molecularly adsorbed acetaldehyde were detected. beta-aldolization of acetaldehyde into crotonaldehyde was observed already at 300 K on all surfaces investigated. The formation of adsorbed crotonaldehyde depended on the temperature and on the nature of the metal. The gas phase products (crotonaldehyde, benzene, hydrogen, ethane, ethylene and acetylene) were analyzed by mass spectrometer. Their appearance in the gas phase and their amounts depended on the nature of the metal and on the temperature.