Two model vanadium-phosphorus-oxygen catalysts, (beta)-VOPO(,4) and (VO)(,2)P(,2)O(,7), were investigated for the selective oxidation of C(,4) hydrocarbons to maleic anhydride. Particular emphasis was given to the characterization of the functioning model catalysts using in situ laser Raman spectroscopy. Complementary techniques including X-ray powder diffraction and X-ray photoelectron spectroscopy were used for characterization of the fresh and used catalytic materials. The in situ Raman technique was particularly valuable since it made possible the characterization of catalyst features during C(,4) hydrocarbon oxidation which were otherwise undetectable by post-catalytic characterization. The direct observation of phase stability and the participation of lattice oxygen during catalysis was possible using the in situ Raman technique. In particular, (beta)-VOPO(,4) and (VO)(,2)P(,2)O(,7) demonstrated bulk structural integrity during n-butane oxidation. The relatively greater reducing capacity of 1-butene induced the (beta)-VOPO(,4) to (VO)(,2)P(,2)O(,7) phase transformation. Direct structural identification of catalytically active centers for paraffin and olefin oxidation were investigated using an ('18)O-enriched (beta)-VOPO(,4) phase catalyst. Active sites responsible for complete combustion (Site I) and selective oxidation (Site II) were identified. The selective route for 1-butene oxidation involved predominantly Site II centers, while Site I centers were associated with complete combustion. In contrast, n-butane oxidation required the highly active Site I centers for initial activation and for the formation of an intermediate containing two oxygen atoms. Raman band assignments indicated these oxygen sites were associated with PO(,4) units in the (beta)-VOPO(,4) lattice;Maleic anhydride conversion was particularly sensitive to the catalytic phase present. Significant combustion activity was observed when maleic anhydride was fed directly to an integral flow reactor charged with model catalysts. The combustion activity was least for (VO)(,2)P(,2)O(,7), nominally a V(IV) phase. The (beta)-VOPO(,4)catalyst, nominally a V(V) phase, resulted in increased conversions of maleic anhydride.