Al-MCM-41 and Cu-Al-MCM-41 were prepared by a modified hydrothermal method. The characterization using XRD, NMR, and N-2 adsorption isotherms revealed that the structure of Al-MCM-41 remained unchanged after the complete exchange of protons for copper ions. Cu-Al-MCM-41 catalysts were studied for the selective catalytic reduction (SCR) of NO by C3H6 in the presence of excess oxygen. Particular attention was paid to the influence of both copper content and Si/Al ratio on the nature of the active copper sites and, therefore, on the performance of the catalysts. It was observed that the most active catalyst have similar to100% of copper exchange. For the underexchanged catalyst of the same copper content, it was found that the NO reduction activity decreased with decreasing Si/Al ratio. H-2-TPR and NO-TPD experiments showed that copper is mainly in the form of isolated Cu2+ ions in Cu-Al-MCM-41 catalysts with copper exchange less than or equal to 100%, whereas at higher copper loadings CuO species are also present. Moreover, when the catalyst Si/Al ratio increases copper is more easily reduced. These results are in agreement with the view accepted in the field of Cu-zeolite catalysts that the isolated Cu2+ ions are involved in the SCR of NO by propylene. (C) 2004 Elsevier Inc. All rights reserved.