A review of literature data for nitric oxide decomposition over Cu-ZSM-5 catalysts leads us to conclude that the turnover frequency depends on the Si/Al atomic ratio in away opposite to the trend suggested by Iwamoto and co-workers and considered correct by Shelef in a recent letter published in this journal. In particular we show that the turnover frequency increases with the number of Al atoms per unit cell (i.e. decreasing the Si/Al atomic ratio). This result suggests that the most active sites for NO decomposition over Cu-ZSM-5 catalysts may contain two close copper ions.