Vanadium/phosphorus mixed oxide catalysts comprising vanadyl pyrophosphate, optionally containing a promoter component, are transformed from vanadium/phosphorus mixed oxide catalyst precursors comprising vanadyl hydrogen phosphate, optionally containing a promoter component, by subjecting the catalyst precursors to elevated temperatures in three stages--(a) an initial heat-up stage in an atmosphere selected from the group consisting of air, steam, an inert gas, and mixtures thereof, (b) a rapid heat-up stage at a programmed heat-up rate in a molecular oxygen/steam-containing atmosphere, and (c) a maintenance/finishing stage, first in a molecular oxygen/-steam-containing atmosphere, and thereafter in a nonoxidizing, steam-containing atmosphere. Such catalysts are useful for the production of maleic anhydride via the partial oxidation of nonaromatic hydrocarbons, particularly n-butane, in the vapor phase with molecular oxygen or a molecular oxygen-containing gas.