The single and combined effect of operating parameters, as water and oxygen concentration, temperature and contact time on the catalytic activity of a VPO catalyst in the oxidation of propane and propylene to acrylic acid has been investigated in a fixed bed reactor. The results indicate that acrylic acid is formed by propylene produced by propane dehydrogenation and that carbon oxides are mainly produced by hydrocarbons oxidation up to 400 degrees C, acrylic acid oxidation becoming significant only at higher temperatures. Water concentrations > 7.5% provide a strong increase of selectivity to acrylic acid at each value of the other operating parameters investigated. Acrylic acid yields can be improved, up to 400 degrees C, by increasing contact time due to the enhancement of propane conversion coupled with quite constant acrylic acid selectivity. (c) 2005 Elsevier B.V. All rights reserved.