Various commercial, homemade, and dealuminated mordenites (MOR) and beta (BEA) zeolites as well as cobalt modified BEAs have been characterized by Si-29- and Al-17 solid-state magic angle spinning nuclear magnetic resonance (MAS-NMR) spectroscopy. The quantitative contributions of Si(nAl) and Si(OH)., sites to the NMR signal intensities were calculated from the various Si/Al ratios and relative Si-29 and Al-17 NMR signal intensities assuming a special distribution of aluminum in different periodical building units of the zeolite framework. A different dealumination and silanol formation mechanism for MOR and BEA was suggested. We conclude that we are able to distinguish the Si(OH), groups that are original defect sites or produced in a dealumination or calcination process.