The dealumination-realumination processes of MOR, MFI, FER and BEA zeolites were investigated by means of XRD, Al-27 MAS NMR, FT-IR and N-2 adsorption. Framework aluminums of BEA zeolite were easily removed from the framework by HCl treatment. Dealumination of MOR zeolite was performed by a combination of calcination in air and HCl treatment. However, in the case of MFI and FER zeolites, although the aluminums were taken out of their frameworks by calcination, they remained in the zeolites even after subsequent HCl treatment. This indicates that the structure type is one of the major factors that influence the dealumination behavior. From realumination of various dealuminated zeolites by pH control in solution, it was also found that aluminum species in the solution, which are eliminated from the frameworks of BEA and MOR zeolites by HCl treatment, are easily reinserted into the frameworks by controlling the pH value of the suspension below 7. The amount of incorporated aluminums increased with an increase in the pH value. However, in the case of MOR zeolite, all of incorporated aluminums were not necessarily responsible for generation of tetrahedral coordinated sites. Reinsertion of non-framework aluminums into the dealuminated MFI and FER zeolites hardly occurred. (C) 2002 Elsevier Science B.V. All rights reserved.