The role of copper species and Bronsted acidity in the selective catalytic reduction (SCR) of NO by propene over CuCl/ZSM-5 catalysts prepared by dispersion method has been investigated. The results of IR and H-2-TPR show that the main active species of CuCl/H-ZSM-5 catalyst are isolated copper ions (Cu2+ and Cu+) located in the ion exchange sites of ZSM-5, which formed during the high-temperature solid-state exchange reaction occurred between CuCl and the Bronsted acid sites of H-ZSM-5. While the high dispersed CuCl on the surface of ZSM-5 is inactive for this reaction. Some direct evidences indicate that the initial Bronsted acidity of the starting zeolite is useful to produce active copper ions species during the preparation process of catalysts. However, it is not necessary for the SCR of NO over Cu-ZSM-5 catalysts. 2002 Published by Elsevier Science B.V.