Aluminosilicate and niobiosilicate ZSM-5 zeolites have been modified by Cu-cation exchange. Structural and physicochemical properties are determined by niobium insertion in the MFI zeolite framework, as shown by TPR, ESR and FTIR investigations. This behavior justifies the different catalytic properties of Cu-NbZSM-5 samples in the NO decomposition with or without the presence of SO2 in comparison with Cu-AIZSM-5.