The heterogeneous catalytic ammoxidation of methyl aromatics and methyl hetero aromatics is a preferred method for the synthesis of aromatic and hetero aromatic nitriles. The resulting nitriles are valuable intermediates for the production of dyestuffs, pesticides, pharmaceuticals and other chemical products. Usually, the ammoxidation is carried out using V-containing oxides (e.g. V/Ti, V/Sn, V/Mo) promoted by further transition metals as catalysts. However, we have shown recently that defined vanadium phosphates (VPO) revealed an excellent ammoxidation performance. The ammoxidation of toluene was used as model reaction. An intense characterisation of some VPO catalysts under pre-treatment and working conditions by the use of various in situ-methods has thrown more light in formation and stability of the used VPO solids as well as the effect of different reactants (ammonia, toluene) on VPO surface and bulk properties.