Incorporation of aluminum atoms into the framework of mordenite type zeolite was investigated using highly dealuminated mordenites and aluminum chloride to clarify the alumination mechanism for highly siliceous mordenite. Mordenite was dealuminated by a combination of calcination and acid reflux to prepare mordenites with various concentrations of aluminum. The concentration of the defect sites estimated from the absorbances of the 3700 and 3500 cm(-1) IR bands due to internal SiOH groups was found to be maximized by the dealumination at Si/Al of 123. The dealuminated mordenites were aluminated by the treatment with AlCl3 vapor at elevated temperatures. By the alumination at 873 K the maximum amount of aluminum was incorporated into the mordenite framework to generate Bronsted acid sites,and only trace amount of silicon was released from the framework. The amount of framework aluminum incorporated by the alumination was proportional to the concentration of the defect sites in parent dealuminated mordenites, which indicates an alumination mechanism where aluminum atoms are incorporated into the tetrahedral sites through the reaction between AlCl3 and the internal SiOH groups. This mechanism was supported by H-1-Si-29 cross-polarization MAS NMR spectra, where the alumination on dealuminated mordenites greatly decreased the signal at -103 ppm attributed to internal SiOH groups. From Si-29 and Al-27 MAS NMR spectra, the stoichiometry of the alumination was estimated, i.e., four internal groups was consumed to form one framework Al site. This confirmed that aluminum atoms were introduced into the defect sites where four internal SiOH groups were clustered as a hydroxyl nest.