A systematic comparison was made of the catalytic activity of a variety of Fe/zeolite catalysts in the selective catalytic reduction of NOx with iso-butane or propane, both with dry and wet feeds. All catalysts were prepared by subliming FeCl3 vapor onto the H-form of the zeolite, followed by Cl removal. The activities decrease in the order Fe/BEA > Fe/MFI >> Fe/FER > Fe/MOR approximate to Fe/Y. These differences are rationalized in terms of our mechanistic model and the known pore geometry of the zeolites. FT-TR shows that some Fe loaded narrow pore zeolites are able to oxidize NO to NO2 and form NOy. chemisorption complexes, but unable to let iso-butane enter the pores. Unlike some wider pore zeolites that become covered by a carbonaceous deposit at low temperature, these materials retain their high activity for NO oxidation. Mixtures of Fe loaded wide and narrow pore zeolites, therefore, lead to higher steady-state activities at low temperature than single catalysts. In particular, the combination of Fe/MFI with Fe/FER displays a spectacular activity enhancement.