From the Al-27 MAS NMR and 2D 3QMAS NMR experiments, it can be concluded that during the steaming process of zeolite omega, some part of the framework tetrahedral Al species are gradually distorted. The 30 ppm broad hump in the ID Al-27 spectra is mainly due to appearance of the distorted tetrahedral Al. With the dealumination process going on, penta-coordinated Al species are formed and contribute also to the 30 ppm broad hump. There exists the trend that the formation of penta-coordinated At occurs with the consumption of the distorted tetrahedral Al species. Because the same phenomenon of the Al-27 NMR signals has also been observed in other zeolites, such as mordenite, ZSM-5 and Y, it could be regarded as a general procedure in the process of dealumination of acidic zeolites. (C) 2004 Elsevier B.V. All rights reserved.