Two-step experiments were conducted to investigate the mechanism of NOx reduction over copper ion-exchanged ZSM-5 zeolites. The first step was the formation of a reactive intermediate and the second one the reaction of this intermediate during a transient regime without any hydrocarbon injection. The presence of NO or NO2 appears to be mandatory to obtain such an intermediate. In contrast, the presence of oxygen is not necessary, The intermediate reacts only with NO2 as well as with O-2. The role of oxygen could be to oxidize NO to NO2 in a first reaction step.