The dynamics of the nitrogen isotopic exchange between adsorption complexes of (NO2)-N-15 and gaseous (NO)-N-14 has been studied by FTIR spectroscopy for Cu/ZSM-5 and Co/ZSM-5. In Co/ZSM-5 the prevailing adsorption complex is a nitrito group ligated to Co2+ 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 Cu2+ ions. The isotope exchange with gaseous NO at room temperature is initially slow, but later autocatalytic. The data suggest that in this case formation of physisorbed NO2 is rate limiting. It rapidly transfers an oxygen atom to impinging gaseous NO. The NO2,phys 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 NO2 + N2O, as was shown previously. Once Cu ligating N-15-nitrate ions are present inside the zeolite, they exchange their (NO2)-N-15 moeity with either (NO2,phys)-N-14 or (NOphys)-N-14. The latter process is limited to the monodentate form, Cu-O-NO2, of the nitrate, and it results in the formation of Cu ligating N-14-nitrite ions and (NO2,phys)-N-15.