AbstractUsing the ESR method the formation of O−and O2−on silica supported vanadiumphosphorus catalysts has been investigated. Studying catalysts with varying phosphorus contents it was found that the concentration of O−and O2−can be related to the contents of phosphate ions. The admixture of increasing amounts of phosphorus obviously increases the reducibility of the catalysts and stabilizes the square‐pyramidical coordination of V4+ions.Comparing these results with activity and selectivity of the catalytic butene oxidation to maleic anhydride we point out that both the changed catalytic properties and the formation of oxygen radicals may be influenced by the same structura