The VPO system, active and selective in the oxidation of butene into maleic anhydride, provides an additional example of a mild oxidation catalyst, where the catalytic properties are shown to depend strongly on the intrinsic properties of the constituent phases. The structural and spectroscopic properties of VOPO 4 and (VO) 2P 2O 7, whose presence is necessary to obtain a substantial selectivity in the range P:V = 0.5:1 to 2:1, have been examined in connection with those of V 2O 5. Since electron diffraction patterns obtained on spent catalyst samples revealed the existence of microdomains, these results have been compared with the hysteresis loop found in kinetic and thermodynamic experiments: The excited atoms, which are located on each side of the coherent boundaries formed in steady state conditions, are assumed to influence strongly the activity and selectivity.