. . . "Distribution of Aluminum in Different Periodical Building Units of MOR and BEA Zeolites" . . . "Various commercial, homemade, and dealuminated mordenites (MOR) and beta (BEA) zeolites as well ascobalt modified BEAs have been characterized by 29Si- and 27Al solid-state magic angle spinning nuclearmagnetic resonance (MAS-NMR) spectroscopy. The quantitative contributions of Si(nAl) and Si(OH)x sitesto the NMR signal intensities were calculated from the various Si/Al ratios and relative 29Si and 27Al NMRsignal intensities assuming a special distribution of aluminum in different periodical building units of thezeolite framework. A different dealumination and silanol formation mechanism for MOR and BEA wassuggested. We conclude that we are able to distinguish the Si(OH)x groups that are original defect sites orproduced in a dealumination or calcination process." . . . . . . . .