Maleic anhydride is produced by the oxidation of 1,3-butadiene, n-butylenes, crotonaldehyde and furan with molecular oxygen in the vapor phase in the presence of catalytic oxides of antimony, molybdenum, at least one element selected from the group consisting of niobium, zirconium, titanium and tantalum and optionally a reducing agent capable of reducing at least part of the molybdenum in the catalyst to a valence state below +6 selected from the group consisting of hydrazine hydrate, molybdenum, tungsten, magnesium, aluminum, and nickel. This catalyst may optionally contain one or more elements selected from the group consisting of Li, Ag, Ce, Cd, Co, As, Si, Zn, Ge, Bi, Ru, Pt, U, Al and Ni. Especially desirable yields of maleic anhydride are obtained from 1,3-butadiene in the presence of a catalyst wherein molybdenum metal is used as a reducing agent.