The influence of the nature of metal ion, framework type, exchange method and exchange degree, the addition of NO2 to the gas flow and the introduction of Al, Nb, and Mn on the selective catalytic reduction (SCR) of NOx with ammonia over ZSM-5 and BEA catalysts was studied before and after hydrothermal aging. From the results, it was concluded that the stability of metal-exchanged zeolites is governed by the framework type, among which Fe-BEA showed superior hydrothermal stability, Cu-ZSM-5 and Fe-ZSM-5 showed medium hydrothermal stability, and Cu-BEA showed relatively low hydrothermal stability. The choice of the exchange metal was a key factor for the activity of the zeolite, whereas Cu showed high low-temperature activity and Fe showed high high-temperature activity in the SCR reaction. Simultaneously exchanged Fe/Cu-ZSM-5 combined the advantages of both exchange metals in one catalyst. The Si/Al ratio did not directly influence the activity or stability. The initial activity was not influenced by the exchange method, but with respect to stability, the chemical vapor ion-exchange method was clearly favored. However, in view of ease of catalyst preparation, liquid phase ion exchange and solid state ion exchange are the preferred methods. By feeding an NO/NO2 = 1:1 mixture, the SCR activity is increased by a factor of about 10-30 for fresh Fe-ZSM-5(w) (0.3) and about 30-100 for aged Fe ZSM-S-w (0.3) compared to only NO. The introduction of extraframework Al in ZSM-5 zeolites and the use of an Fe3+ metal salt as a precursor improved the stability of the Fe-ZSM-5 zeolites against hydrothermal aging in terms of the SCR activity. On the other hand, neither the introduction of Nb or La nor the simultaneous exchange of Mn and Fe enhanced the NO conversion after aging.