A process for the manufacture of a phosphorus-vanadium and phosphorus-vanadium co-metal catalysts suitable for use in the manufacture of maleic anhydride from benzene, butane, and other C.sub.4 hydrocarbons which process comprises reacting at a temperature of about 0.degree. C. to about 200.degree. C. a vanadium compound in an organic ether solvent having about 2 to about 10 carbon atoms with a phosphoryl halide in the presence of water or an aliphatic alcohol having from about 1 to about 8 carbon atoms, eliminating the solvent and activating the catalyst by the addition of the hydrocarbon feedstock and water and a phosphorus compound at a temperature of about 300.degree. C. to about 500.degree. C. wherein the amount of water added is about 1000 parts per million to about 40,000 parts per million by weight of the reactor feed gas stream is disclosed. The catalysts are useful for the manufacture of maleic anhydride from butane, benzene, and other C.sub.4 hydrocarbon feedstocks.