Abstract: An unsaturated normal aliphatic hydrocarbon of 4 or 5 carbon atoms and an oxygen-containing gas react to form an effluent containing maleic anhydride as a result of a reaction in the presence of a catalyst consisting essentially of a mixture of the oxides of tungsten, phosphorus, a Group A metal, and a multivalent metal having an atomic number from 21 to 47, the sum of the oxides of the Group A metal and the multivalent metal constituting from 1% to 15% of the catalyst, the atom ratio of the Group A metal to the transition metal being within a range from about 1:1 to about 8:1 and the overall atomic ratios are such that W:P:A:M corresponds to 1:0.1-0.7:0.01-0.16:0.01-0.1. In preferred embodiments the reaction is conducted in the presence of from about 3% to about 50% by volume of steam, desirably from about 5% to about 7% by volume steam, whereby catalyst stability and selectivity are enhanced.