In this letter we present a simple model useful to understand the relationship between the turnover frequency for NO decomposition over Cu-ZSM-5 catalysts (N(Cu)), the number of Al atoms per unit cell of the ZSM-5 zeolite (p), and the copper loading expressed as percent of exchange (E). Our simple model is able to explain the literature data. We show that : (1) on catalysts with the highest activity (Cu exchange levels E>90%), N(Cu) increases with p (i.e. decreasing the Si/Al ratio) indicating that the most active sites may contain two close copper ions; (2) at low Cu exchange levels (E<80%) the catalysts have lower activity and, moreover, N(Cu) decreases with p, according to previous results of Iwamoto et al. (1986). The present results are also in agreement with the evidence that the redox couple Cu2+/Cu+ play a key role in the reaction mechanism.