The selective oxidation of n-butane to maleic anhydride (MA) on a vanadium-phosphorus oxide (VPO)catalyst was studied using on-line gas-chromatography combined with mass spectrometry(GC-MS) and transientresponse technique. The reaction intermediates, butene and furan, were found in the reaction effluent under nearindustrial feed condition (3% butane+15%O2), while dihydrofuran was detected at high butane concentration (12%butane, 5%O2). Some intermediates of MA decomposition were also identified. Detection of these intermediatesshows that the vanadium phosphorus oxides are able to dehydrogenate butane to butene, and butene further to formMA. Based on these observations, a modified scheme of reaction network is proposed. The transient experimentsshow that butane in the gas phase may directly react with oxygen both on the surface and from the metal oxidelattice, without a proceeding adsorption step. Gas phase oxygen can be adsorbed and transformed to surface latticeoxygen but it can not participate in selective oxidation. Adsorbed oxygen leads to deep oxidation, while latticeoxygen leads to selective oxidation.