This study concerns the generation of the active V/P/O phase in supported catalysts used for n-butane oxidation to maleic anhydride Catalysts were prepared by impregnating a zirconia support with ammonium vanadate and phosphoric acid with subsequent thermal treatment under various environments and temperatures The catalysts were studied by means of In situ Raman spectroscopy and reactivity measurements It was found that the nature of the active surface is greatly affected both by the composition of the gas phase used for the thermal treatment and by the P/V ratio used for catalyst preparation delta-VOPO4 was initially formed by the high-temperature reaction of vanadium salt with phosphoric acid however in the sample prepared with only 10% excess phosphorus (P/V atomic ratio = 1 1) delta-VOPO4 transformed into either V2O5 in a steam-enriched air stream or alpha(II)- and beta-VOPO4 in a dry air stream delta-VOPO4 was more stable in the sample prepared with a large excess of P i e with P/V = 1 3 and 1 5 The latter catalyst showed the best catalytic performance in n-butane oxidation However the performance was worse than that of the commercial unsupported vanadyl pyrophosphate catalyst (C) 2010 Elsevier B V All rights reserved