We prepared H-ZSM-5 (Si/Al=66), Fe-S-1 (Si/Fe=66), and Silicalite-1 (S-1) according to the methods described by Bellussi and coworkers, and compared their ion-exchange capacity for copper ions. The Cu-ZSM-5, Cu-Fe-S and Cu-S-1 samples thus obtained have been investigated as catalysts for the NO decomposition reaction at 773K. We found that using copper acetate solutions with concentrations in the range 0.004M=<[Cu 2 + ]=<0.1M, at room temperature and at 323K, it is very easy to prepare overexchanged Cu-ZSM-5, Cu-Fe-S-1, and Cu-S-1 catalysts. XRD and Vis-UV DRS techniques show that after thermal treatments of the fresh samples in air at 823K for 4h no segregation of bulk CuO phase occurs, suggesting the presence of low nuclearity [Cu n O x (OH) y ] q + species [q=2(n-x)-y=<0]. These results were confirmed by TPR studies. At 773K, only Cu-ZSM-5 and Cu-Fe-S-1 catalysts showed NO decomposition activity (NO 1% in He, W/F=0.2gscm - 3 ). Instead, the activities of Cu-S-1 and of the copper-free matrices (S-1, H-ZSM-5 and Fe-S-1) were not measurable under our experimental conditions. The negative results obtained with Cu-S-1 catalysts clearly demonstrate that the active sites for NO decomposition in Cu-MFI catalysts consist of polynuclear ionic copper species strongly anchored to framework [MO 4 ] - species (M=Al, Fe).