The redox properties of vanadium phosphorus oxide (VPO) catalysts have been compared to their catalytic performance in fuel-lean conditions for two sets of (VO)(2)P2O7 catalysts obtained after calcination under nitrogen at different temperatures or after different postoxidation periods under dioxygen. It is shown that doping the VPO catalysts with Co and Mo makes possible for the catalyst to produce maleic anhydride in fuel-rich conditions. In both situations, catalytic performances are dependent on the vanadium oxidation state of the catalysts.