The oxidative dehydrogenation of isobutyric acid (IBA) to methacrylic acid (MAA) has been studied over cupric salts of undecamolybdovanadophosphoric acid CuxH4-2xPVMo11O40 The presence of copper as counter cation in heteropoly compounds increases the activity and the selectivity to MAA. The best catalytic performance is obtained for s=0.5-0.7. Samples were characterized by V-51 solid state NMR, EPR, XPS, IR and elementary analysis in order to obtain information on the nature and location of vanadium and copper species in the sample before and after reaction and on the possible changes occurring during the course of the catalytic reaction. It is shown that vanadium localized initially in the heteropolycompound is removed from the Keggin Unit at 593 K, Small clusters of vanadium and copper oxide species with unsaturated coordination are formed. The reduction of V5+ into V4+ is also evidenced during the catalytic test. Delocalized electrons appear in these conditions inside the small vanadium and copper oxide clusters. This behavior of vanadium and copper is correlated with higher performance of this catalyst, and then appears as a synergic effect. The stability of Cu0.5H3PVMo11O40 catalyst was also tested. After 250 days of catalytic test, some deactivation was pointed out. This effect is related to a molybdenum loss followed by important surface aggregation of copper. Electronic and ionic exchange are then disturbed in the core of the solid and therefore the reoxidation of the catalytic system is hindered.