FTIR analysis of the gaseous products of the selective catalytic reduction of NO by isobutane over a Fe-MFI catalyst made by solid-state ion exchange shows, for the first time, that HCN can be a substantial product. Under dry conditions the amounts formed can exceed that of N-2 for temperatures up to 280 degrees C and NO conversions of 25% with a maximum HCN concentration of similar to 150 ppm formed at 315 degrees C. However, introduction of 0.7% water causes the subsequent conversion of about one-half the HCN to N-2, probably through hydrolysis to NH3 and the NH3-SCR reaction which is very rapid on Fe-MFI. The steps through which HCN forms remain to be established.