n-Butane partial oxidation to maleic anhydride is practiced commercially in several reactor types with vanadium phosphorous oxide (VPO) catalyst. DuPont operated a Circulating Fluidized Bed facility in which catalyst was shuttled between a net reducing zone (a transport bed) to a net oxidizing zone (air regenerator). Several advantages have been cited for this technology but the role of oxygen is hotly debated and different models have been proposed to characterize the complicated 14e(-) exchange. To examine the role of surface lattice oxygen, we carried out transient experiments in which catalyst was subjected to high concentrations of butane followed by an extensive re-oxidation treatment. Carbon accumulates on the surface lattice surface thereby contributing to a reduction in reaction rates and the quantity increases with the butane/oxygen ratio. This carbon reacts in the presence Of Molecular O-2 during the regeneration step. The surface lattice is capable of storing significant amounts of oxygen and when the catalyst is highly oxidized, both selectivity and activity are higher. (c) 2005 Elsevier B.V. All rights reserved.