Effect of water on the catalytic performances of VPO catalyst in the oxidation of propane to acrylic acid has been investigated at 400 degreesC carrying out both stationary and dynamic tests at different water vapour concentrations. Presence of water, necessary to obtain a detectable formation of acrylic acid, affects, in addition to the catalytic performances, both crystallinity and surface acidity of VPO leading to slow and reversible modifications of the material as shown by XRD and NH3 TPD analysis. A maximum acrylic acid yield was obtained in correspondence of 15% H2O. Under this condition, the sample shows the best crystallinity and the highest surface acidity among catalysts operated under wet conditions. A linear correlation between kinetic constant of acrylic acid formation and acid sites was found. (C) 2004 Elsevier B.V. All rights reserved.