This study focuses on the mechanism of lean NO, reduction by propane, over acidic zeolites (HZSM-5), and the influence of surface acidity. In situ FTIR measurements of ammonia adsorption indicate a higher number of Bronsted acid sites for a sample with a low SiO20/Al2O3 ratio. The activity for NOx reduction and the selectivity for N-2 formation correlate well with the Bronsted acidity. Step-response experiments with NO2 and propane show the formation of surface-bound NO+, isocyanate, unsaturated hydrocarbons, and amine species. Formation of the latter two seems to be closely related to the Bronsted acidity. In the NO2 reaction with propane, the NO+ species seem to play a vital role, probably reacting with carbenium ions (from propane cracking) to form isocyanates, which may be hydrolysed to airline species. Step-response experiments with isopropylamine and NO2 indicate a fast reaction, where the amine and NO+ species react over Bronsted acid sites. Hence, amine species are possible reaction intermediates in the lean reduction of NO2 by saturated hydrocarbons, such as propane, over HZSM-5. © 2005 Elsevier Inc. All rights reserved.