The influence of the metal in Ni-containing zeolites used as catalysts for the reduction of NO with propane and propene was studied. In the fresh catalysts, Ni is located in ion exchange positions for Ni/MOR, Ni/ZSM-5, and Ni/MCM-22. The formation of carbonaceous deposits, the removal of Al from framework positions, and the migration of Ni ions to nonaccessible positions were identified as primary reasons for the changes in the activity of the catalysts during the reduction of NO with hydrocarbons. Changes in the chemical state of Ni, i.e., isolated Ni2+ species being converted into Ni2+ on the surface of small Ni oxide clusters, affect the activity to a less significant extent compared to the changes in the location of Ni within the zeolite framework. (C) 2003 Elsevier Inc. All rights reserved.