In order to understand the role of different aluminum species in the selective catalytic reduction of nitrogen oxides with methane over H-mordenites, solids with varying Si/Al ratios (5.9-16.9) were prepared by acid leaching. They were thoroughly characterized before and after reaction. The distribution of Al was determined through Al-27 MAS NMR. All the samples presented three signals, one at 54 ppm corresponding to lattice Al-IV, another at 0 ppm associated with octahedrally coordinated Al, and a broad band, BB (ca 100 ppm wide), assigned to aluminum containing species. As the spinning rate increased up to 11.3 kHz, a decrease of the BB intensity and an increase of the Al-IV signal took place, while the Al-VI slightly increased. The best estimate of lattice aluminum was obtained from the Al-IV peak intensity. Despite the high spinning rate employed, it was possible to observe only between 70-80% of the total Al present in the samples. The catalysts were also analyzed by XRD, FTIR, and Xe-129 NMR of physisorbed Xenon, By correlating the variation of the a cell constant with Al/u.c., only qualitative information was obtained. The IR band shift at ca 572 and 588 cm(-1) at higher wave lengths, and the decrease of the bands intensity at 650 and 730 cm(-1) with decreasing Al content were examined. These changes in the IR spectra are a clear indication of the dealumination process carried out in the samples, thus supplementing the Al-27 MAS NMR results and supplying information on the dealumination mechanism as well. Xe-129 NMR results shows that nonlattice aluminum may interrupt the free exchange of molecules between the main channels and side pockets. The turnover frequency of NO disappearance remains constant with varying lattice aluminum content. The catalysts were partially deactivated after being on stream at 650 degrees C due to the additional dealumination occurring at high temperatures in the reacting stream. Both in fresh and used catalysts, only the sites related with lattice aluminum were active in the reaction under study. The nonlattice, polymeric species, generated during dealumination hinder the access of the reactant molecules to the active sites.