In this study, the selective catalytic reduction (SCR) of NO2 by two saturated hydrocarbons, propane and isobutane, has been investigated over acidic zeolite HZSM-5. The activity for NO2 reduction and the selectivity for N-2 formation correlate well with the intracrystalline acidity (Bronsted acid sites) of the zeolite. Isobutane is a more efficient reducing agent than propane due to its tertiary hydrogen being more readily activated than the secondary hydrogen in propane. In situ DRIFT step-response experiments with NO2 and isobutane show formation of surface bound NO+, isocyanate, unsaturated hydrocarbons and amine species. The NO+ species seems to play an important role in the reduction of NO2 probably by reacting with alkenes formed from carbenium ion adsorbates. These species likely form isocyanates, amine species and finally nitrogen. (c) 2006 Elsevier B.V. All rights reserved.