The adsorption of crotonaldehyde and maleic anhydride on MoO3, respectively, V2O5 has been studied by FTIR spectroscopy. Crotonaldehyde was adsorbed mainly in molecular form due to hydrogen bonds with Bronsted acid sites of these oxides (physical adsorption) at room temperature (RT). Stable surface complexes (coordinatively bonded crotonaldehyde, carbonyl compounds and carboxylates) were found during the adsorption of crotonaldehyde on MoO3, respectively, V2O5 at 298-573 K. Carboxylates and consecutively maleic anhydride were products of reaction between crotonaldehyde and active sites Of V2O5 surface at high temperatures in the atmosphere of reaction gas. Maleic anhydride was physically adsorbed on MoO3, respectively, V2O5 via hydrogen bonds with surface hydroxyls. (C) 2004 Elsevier B.V. All rights reserved.