Abstract: The performance of phosphorus-vanadium-oxygen catalysts useful for the conversion of saturated aliphatic hydrocarbons to maleic anhydride is dependent on the controlled calcination of the catalyst precursor. Phosphorus-vanadium-oxygen catalysts are prepared by mixing phosphorus and vanadium compounds to form precursors containing tetravalent vanadium, and calcining the precursors in an oxidizing atmosphere until a controlled level of pentavalent vanadium has been achieved, and then calcining the precursors in an inert atmosphere to higher temperatures to provide catalysts with the desired oxidation levels that are promptly active, so that valuable manufacturing facilities are not required to "activate the catalyst.