ZSM-5 containing Zr in the lattice was prepared following the procedure for ZSM-5 using ZrCl4 as zirconium precursor. Zr was substituted for Si in the ZSM-5 lattice and then this zeolite was loaded with Cu. Loading these catalysts with Cu was carried out following Iwamoto＇s procedure. An increase in Zr results in a decrease in Cu loading, although the exchange solutions were the same. The increased Zr loading and, hence, decreased Cu loading resulted in a decreased NO decomposition activity relative to the Zr-free ZSM-5 loaded with Cu. NO conversions, obtained on Cu-ZSM-5 zeolites in which zirconium replaced Si, were below 40%, which is about 30% lower than those recorded on Cu-exchanged pure zeolites. The amount of NO2 detected in the reaction products proved that, for total conversions lower than 20%, the reaction of NO takes place almost exclusively without decomposition to N-2 and O-2. The catalysts were characterized by DRIFTS and NO-DRIFTS, Raman spectroscopy, O-2 - TPD and XPS. The OH region observed in DRIFTS changed with an increase in Zr loading and, hence, Cu(II)-O-Cu(I) species cannot be formed. This may play a role in the decreased activity. Raman spectra showed that increased Zr also results in increased CuO which may lead to the decreased activity. Although the exposure of the samples with high Zr content to NO results in the apparent redispersion of CuO to isclated sites within the ZSM-5, the Cu moves to locations which are inactive and/or inaccessible to NO and, hence, the activity does not increase even though the Cu is dispersed. The formation of NO2 follows the mechanism proposed by Shelef. (C)1999 Elsevier Science B.V. All rights reserved.