The selective oxidation of n-butane to maleic anhydride was studied using two model vanadium-phosphorus-oxygen catalysts, ..beta..-VOPO/sub 4/ and (VO)2P2O/sub 7/. Detailed synthesis and characterization of these phases are reported, demonstrating that synthesis of the pure compounds is very sensitive to preparation procedures. New information regarding the reaction pathway is presented with the detection of 1-butene, 2-butenes, and 1,3-butadiene as potential reaction pathway intermediates. 43 references, 42 figures, 18 tables.