The dynamics of the nitrogen isotopic exchange between adsorption complexes of O 1 5 NO 2 and gaseous O 1 4 NO has been studied by FTIRspectroscopy for Cu/ZSM-5 and Co/ZSM-5. In Co/ZSM-5 the prevailing adsorption complex is a nitrito group ligated to Co 2 + ions, Co-ONO; it exchanges its nitrogen atom spontaneously with gaseous NO. In Cu/ZSM-5 nitro groups and nitrate ions prevail that are ligated to Cu 2 + ions. The isotope exchanges with gaseous NO at room temperature is initially slow, but later autocatalytic. The data suggest that in this case formation of physisorbed NO 2 is rate limiting. It rapidly transfers an oxygen atom to impinging gaseous NO. The NO 2 , p h y s is formed over pre-calcined Cu/ZSM-5 by oxygen transfer from [Cu-O-Cu] 2 + ions to NO, which is also the first step in the decomposition of NO into NO 2 + N 2 O, as was shown previously. Once Cu ligating -nitrate 1 5 N-nitrate ions are present inside the zeolite, they exchange their O 1 5 NO 2 moeity with either O 1 4 NO 2 , p h y s or O 1 4 NO p h y s . The latter process is limited to the monodentate form Cu-O-NO 2 , of the nitrate, and it results in the formation of Cu ligating -nitrite 1 4 N-nitrite ions and O 1 5 NO 2 , p h y s .