The oxidation of NO and of SO2, separately and together, has been investigated over a Fe-MFI catalyst made by solid-state ion exchange. The catalyst is more active than Pt/SiO2 for the oxidation of NO alone but less active for SO2 oxidation. The activity for NO oxidation is greatly inhibited by the simultaneous presence of SO2 or water but complete recovery of the activity is possible when they are removed subsequently. Exposure to H2O during NO oxidation results in displacement of NO2 and NO to an amount which is equivalent to a NOx/Fe ratio of 2. A Fe-ferrierite catalyst also made by solid-state ion exchange is more active that Fe-MFI for NO oxidation but more susceptible to SO2 poisoning.