The catalytic selective reduction of NO over Cu-exchanged natural zeolites (mordenite (MP) and clinoptilolite (HC)) from Cuba using NH3 as reducing agent and in the presence of excess oxygen was studied. Cu(II)-exchanged zeolites are very active catalysts, with conversions of NO of similar to95%, a high selectivity to N-2 at low temperatures, and exhibiting good water tolerance. The chemical state of the Cu(II) in exchanged zeolites was characterized by H-2-TPR and XPS. Cu(II)-exchanged clinoptilolite underwent a severe deactivation in the presence of SO2. However, Cu(II)-exchanged mordenite not only maintained its catalytic activity, but even showed a slight improvement after 20 h of reaction in the presence of 100 PPM of SO2. (C) 2004 Elsevier B.V. All rights reserved.