The selective oxidatiou of n-butane to maleic allhydride (MA) oll a vanadium-phosphorus oxide (VPO) catalyst was studied using on-line gas-chronlatogr&phy combined with mass spectrometry (GH-MS) and transient response technique. The reaction intermediates, butene and furan were found ill the reaction elfluent under near industrial feed condition (3% butane+15% O2), while dihydrofuran was detected at high butane concentratiori (12 butane. 502). Some intermediates of MA decomposition were also identified. Detection of these intermediates shows that the vanadium phosphorus oxides are able to dehydrogenate butane to butene, and butene furl her to form MA. Based on these observations, a modified scheme of reaction network is proposed. The transient expetrinents show that butane in the gas phase may directly react with oxygen both on the surface and from the metal oxlde lattice, without a proceeding adsorption step. (Gas phase oxygen can be adsorbed and transformed to surface lattice oxygen but it can not participate in selective oxidation. Adsorbed oxygen leads to deep oxidation, while lattice oxygen leads to selective oxidation.