The effect of structure activity relationships for vanadium phosphorus catalysts for the oxidation of butane to maleic anhydride are described and discussed. A range of characterisation techniques including in situ laser Raman spectroscopy and transmission electron microscopy have been used on catalysts prepared by using three different methods : (i) VPA, prepared by using the standard aqueous HCl method followed by a water extraction step; (ii) VPO, prepared by the reaction of V2O5 with H3PO4 in isobutanol followed by a water extraction step; and (iii) VPD, prepared by the reaction of VOPO4 . 2H(2)O with isobutanol. The key results indicate that:(a) The active site can be formed on a wide range of vanadium phosphorus containing phases due to the observation that VPA, VPO and VPD catalysts with different compositions give similar specific activity for maleic anhydride formation.(b) The in situ LRS and TEM studies show that the hemihydrate to final catalyst transformation is complex and that after a short exposure to butane/air at temperatures >370 degrees C the hemihydrate is transformed mainly to a disordered material. At the onset of maleic anhydride formation, there is a complex mixture of phases present, including (VO)(2)P2O7 and a variety of VOPO4 phases.(c) For Co2+ promoted by VPO and VPD catalysts, the activity for maleic anhydride formation is a function of the concentration of the V(V) phases present.From these studies, we conclude that the active center for butane activation and maleic anhydride formation comprise a V4+/V5+ couple that is well dispersed on the surface of a range of VPO phases, which for well-equilibrated catalysts will be (VO)(2)P2O7. This hypothesis is discussed in relation to other recent data which also indicate the importance of V5+ centers in the selective activation of butane to maleic anhydride.