Hydrothermal stability of mordenite type catalysts including synthetic (CuHM) and natural (CuNZA) zeolites has been examined under a simulated lean NOx wet condition. When the catalysts were hydrothermally aged at 800 degrees C with 10% H2O for 6 h, the NO removal activity at the reaction temperature of 450 degrees C was higher for natural zeolite than for synthetic one. It was mainly due to the Si/Al ratio of the catalyst. CuHM and CuNZA catalysts were dealuminated to investigate the effect of Si content of the zeolites on the hydrothermal stability. After the aging at 800 degrees C for 24 h in the presence of 10% H2O, NO removal activity of the dealuminated CuHM catalysts at the reaction temperature of 500 degrees C was in the order of the Si/Al ratio of the catalysts. Such a significant improvement of the deNO(x) performance was also observed for the dealuminated CuNZA catalysts. It reveals that the Si/Al ratio of zeolite catalysts is one of the crucial characteristics enhancing the water tolerance and hydrothermal stability of the catalysts for NO reduction under the lean NOx wet condition. In addition, the loss of NO removal activity of the catalysts upon the hydrothermal aging is mainly attributed to the chemical alteration of Cu2+ ions on the catalyst surface through the structural collapse of the zeolite catalysts. (C)1999 Elsevier Science B.V. All rights reserved.