A vanadium phosphate (VPO) catalyst promoted with indium and tetraorthosilicate (TEOS) was calcined in atmospheres of different oxidizing potential. The resulting catalysts after activation showed significantly different stable activity and selectivity. The vanadium valence of the used catalyst increased with increasing oxidizing potential of calcining gases. The selectivity to maleic anhydride and specific activity increased with the vanadium valence, but the surface area decreased with the vanadium valence. Amorphous V5+ species in the working catalyst was readily hydrated. The phase evolution during the thermal dehydration of the hydrate was similar to that of alpha-VOPO4 . 2H(2)O. The oxidation reaction did not result in surface enrichment of V5+ relative to V4+. The calcination atmospheres affect the catalyst morphology and the vanadium valence. An unpromoted VPO catalyst was more resistant to oxidation than the promoted catalyst in calcination, and achieved its optimized maleic anhydride yield when calcined in a more oxidizing atmosphere. The study suggests that dispersed V5+ species in contact with (VO)(2)P2O7 is important for the oxidation of n-butane to maleic anhydride. Calcination atmospheres need to be chosen in accordance with the oxidizability of the VPO catalyst precursors to achieve the optimized performance.