Starting from VO(HPO4), 0.5 H2O a VPO catalyst has been activated at 400 degrees C in an n-butane/air mixture. The catalyst has been characterized at different activation times (up to 132 h) by XRD, P-31 NMR (spin-echo mapping and magic angle spinning) and XPS. A quantitative estimation of the amount of V5+ has been performed in the bulk (by NMR) and on the surface (by XPS) of the catalyst. During activation, the VPO catalyst undergoes a progressive reduction (into (VO)(2)P2O7) of the delta-VOPO4 phase, which is also transformed in part into alpha(Pi)-VOPO4. These changes have been correlated with the evolution of the catalytic performances. The n-butane conversion and the selectivity for maleic anhydride increase with the V4+/V5+ surface ratio. The results have been discussed in relation to the relevant published studies. It is concluded that the dynamic surface concentration of V5+ species, which are acting as oxidizing centers, is reduced during activation to a level low enough to achieve an efficient VPO catalyst.