Abstract: A method of producing maleic anhydride by oxidizing a hydrocarbon material selected from unsaturated hydrocarbons having four or more of carbon atoms, butane and mixtures thereof with molecular oxygen or a molecular oxygen containing gas in gaseous phase and in the presence of a catalyst. The catalyst used in this method is of the type which contains vanadium, phosphorus, potassium and iron, and these components are contained in the following ratios per atom of vanadium: phosphorus, 2/3 to 140 atoms; potassium, 1/4 to 2 atoms; and iron, 1/10 to 70 atoms. Preferably the atomic ratio of phosphorus/iron is greater than 1.