The performance of Cu-ZSM-5 and Co-ZSM-5 zeolites for selective catalytic reduction (SCR) of nitric oxide (NO) was examined and compared in a packed-bed tubular reactor using methane and propene as reducing agents. Emphasis was given to the effects Of SiO2/Al2O3 ratio and Cu or Co ion loading on the catalyst performance as indicated by the levels of NO conversion into N-2. For Cu ion-exchanged zeolites, the catalysts with the same copper loading but different SiO2/Al2O3 ratios showed different activities. At temperatures greater than 350 degreesC, the NO conversion with increasing SiO2/Al2O3 ratio, whereas the maximum NO conversion decreases with increasing copper loading when the catalysts have the same SiO2/Al2O3 ratio. Catalysts with identical Cu ion-exchange levels, but different SiO2/Al2O3 ratios and different copper loadings, do not show the same activities, indicating the important role Of SiO2/Al2O3 ratio. For Co ion-exchanged catalysts with the same Co loadings, however, the activity increases with decreasing SiO2/Al2O3 ratio. For catalysts with the same SiO2/Al2O3 ratios, the catalytic activity decreases with increasing Co loading. Performance of Co ion-exchanged catalysts also appears to be dependent on the preparation method.