Transient experiments in vacuo applying the Temporal-Analysis-of-Products (TAP) reactor were carried out to elucidate the role of oxygen in the reaction chain from n-butane to maleic anhydride over a quasi-equilibrated vanadyl pyrophosphate catalyst. The oxidation of butane and the intermediates butadiene and furan using O-18(2) as oxygen source showed that a redox mechanism operates in all reaction steps including the conversion of furan to maleic anhydride. The formation of CO2, however, is explained by both a redox mechanism and a mechanism in which adsorbed oxygen species are involved.