The decomposition of NO over four Co-containing ZSM-5 zeolites and Pr-, Ga-, and Cu-exchanged ZSM-5 zeolites was investigated using the isotope labeled (NO)-N-15-O-18 and a temperature-programmed desorption (TPD) technique. We found that [Co]-ZSM-5 that contains Co in the framework had the highest activity for NO decomposition (TOF : 2.05 x 10(-3) s(-1) at 442 degrees C and 0.2 vol% NO and 0.8 vol% O-2), almost an order of magnitude greater than that previously reported for a zeolite catalyst, namely Cu-ZSM-5 (TOF : 2.27 x 10(-4) s(-1) at 335 degrees C and 0.2 vol% NO and 0.8 vol% O-2) obtained under steady-state conditions. The phenomenally high activity of [Co]-ZSM-5 is due to the unique incorporation of Co2+ in the siliceous MFI structure. For all the catalysts investigated, coadsorption of NO and O-2 led to a substantial increase in the amount of NOx adsorbed. However, the adsorbed species were not necessarily NO2 as reported by others. We believe that the interaction between adsorbed NOx species and O-2 is responsible for enhancing the rate of NOx decomposition. It is obvious that the framework Co2+ behaves very differently from Co2+ in the countercation position and from extra-framework CoO Such as that supported on or dispersed on the surface of silicalite also having the same MFI structure.