n-Butane selective oxidation over the VPO catalyst to maleic anhydride is the first and only commercialized process of light alkane selective oxidation. The mechanism of this reaction is still not well known despite over twenty years of extensive studies, which can be partially attributed to the extreme difficulties to characterize catalytic reactions real-time under typical reaction conditions. In situ spectroscopic characterization techniques such as infrared spectroscopy and laser Raman spectroscopy were used in the current mechanistic investigations of n-butane oxidation over VPO catalysts. To identify the reaction intermediates, oxidation of n-butane, 1,3-butadiene and related oxygenates on the VPO catalyst were monitored using FTIR spectroscopy under transient conditions. n-Butane was found to adsorb on the VPO catalyst to form olefinic species, which were further oxidized to unsaturated, noncyclic carbonyl species. The open chain dicarbonyl species then experienced cycloaddition to form maleic anhydride. VPO catalyst phase transformations were investigated using in situ laser Raman spectroscopy. During reduction-oxidation step changes, (VO)2P2O7 was readily converted to αII-, δ-VOPO4 and ultimately to β-VOPO 4 in O2/N2; these V5+ phases were eliminated in n-butane/N2. A “wet” nitrogen feed (5–10% H2O in N2) transformed (VO)2P2O 7 and αI-, αII-, β-, δ, γ-VOPO 4 to V2O5 at temperatures above 400°C. The presence of water vapor facilitated the loss of oxygen atoms involved in V-O-P bonding; separated vanadium oxide and phosphorus oxide species were formed. The isolated vanadium oxides than transformed to V2O5, and phosphorus species likely diffused from the catalyst lattice in the form of acid phosphates.