The development of novel and improved microstructures is reported for vanadium phosphorus oxide (VPO) catalysts for the oxidation of n-butane to maleic anhydride (MA) by grafting alkoxides of bismuth and molybdenum on to pre-formed vanadium phosphorus oxide precursor, VO(HPO4)(.)0.5 H2O. Depth profiling shows that the surface is molybdenum-rich, following a calcination and activation procedure which converts the catalyst precursor to the final, active phase(s). This novel composition profile in the catalyst may be responsible for the reactivity enhancements observed for this catalyst system. Catalysts prepared using bismuth or molybdenum containing precursors individually do not show a performance enhancement. (C) 2004 Elsevier B.V. All rights reserved.