The oxidation kinetics of partially reduced vanadyl pyrophosphate (VPO) catalyst were investigated with a novel oscillating microbalance reactor, The oxidation rate of partially reduced VPO catalyst with oxygen is close to half-order with respect to oxygen partial pressure. The activation energy for this oxidation is 70 kJ mol(-1). The kinetics obtained from the transient reaction study are applicable in the design and optimization of the circulating fluidized bed riser reactor process for the selective oxidation of butane to maleic anhydride.