Abstract X-ray photoelectron spectroscopy (XPS) in situ electron paramagnetic resonance (EPR), in situ Fourier transform infrared spectroscopy (FT-IR), SO2 temperature-programmed desorption (TPD), and reaction tests were performed to investigate the poisoning effect on the selective catalytic reaction of NO by i-butane over Fe/ZSM-5. Kinetic studies at 623 K, showed that the presence of SO2 in the reaction mixture poisoned Fe/ZSM-5 irreversibly. XPS studies of Fe/ZSM-5 after reaction with SO2 at 623 K revealed the formation of sulfate species, which corresponds well to the SO2 TPD result. in situ EPR spectra showed that the lines arising from the distorted tetrahedral Fe3+ ions at g≈5.8 and 6.5, which were known to be reactive species, disappeared and a new and sharp one at g≈1.9978 assigned to paramagnetic coke species appeared. The preadsorption of SO2 suppressed the formation of the IR bands arising from Fe–NO2 (1623 cm−1) and Fe–NO3 (1573 cm−1). On the basis of these results, SO2 deactivated Fe/ZSM-5 by suppressing the formation of Fe–NOy (y=2, 3) complexes and promoting the formation of carbonaceous deposits, which resulted in consumption of the active Fe3+ sites.