The selective oxidation of n-butane to maleic anhydride (MA) on (VO)(2)P2O7-based catalysts having different vanadium oxidation states (+3.18 to +4.90) was studied by a vacuum transient technique applying the Temporal-Analysis-of-Products (TAP) reactor. Experiments in the absence of gas-phase oxygen (anaerobic conditions) revealed that all steps of the oxidation to MA may proceed on V5+ as well as on V4+ sites and that the step from furan to MA needs the catalyst state with the highest oxidation potential. In equilibrated (VO)(2)P2O7 catalysts the valence state of the vanadium in the surface layers involved in the selective reaction amounted to +4 and +5. Under such conditions all reaction steps primarily occur on V5+ centers but a contribution of the redox couple V4+/V3+ in these steps cannot be excluded.