The reduction of NO over Ni-exchanged mordenite in the presence of excess oxygen using propane and propene as reducing agents was studied. During ion exchange of the zeolite with Ni2+, Bronsted acid sites were formed resulting from the hydrolysis of divalent Ni2+ ions. The chemical state of the Ni ions and the density and strength of the acid sites strongly determine the activity and selectivity of the catalysts for the NO reduction. Non-reduced catalysts showed a high selectivity to N-2, while after partial reduction of the Ni ions significant concentrations of NO2 and N2O were formed. In addition, low concentrations of NO2 were formed during reduction of NO with propane and related to the more demanding activation of the alkane. An optimum concentration of acid sites, which in turn depends on the nickel loading, is required to obtain high NO conversion with propane. Deactivation due to deposition of carbonaceous species was observed during the NO reduction with propene for samples with high concentration of acid sites. (C) 2002 Elsevier Science B.V. All rights reserved.