Catalysts useful for the partial oxidation of non-aromatic hydrocarbons, particularly n-butane, with molecular oxygen or a molecular oxygen-containing gas in the vapor phase to produce maleic anhydride are provided which comprise mixed oxides of phosphorus and vanadium. The catalysts are highly effective in that they exhibit a weight/weight productivity of at least 70 grams of maleic anhydride per kilogram of catalyst per hour.