Two zeolites of different topology: ZSM-5 and ZSM-12 were subjected to desilication in the NaOH solutions of increasing concentrations. Changes in the properties of modified zeolites were investigated by several methods ((29)Si and (22)Al MAS NMR. FTIR, SEM, XRD, and sorption techniques). It was shown that desilication led to the formation of mesopores, and indicated that the process started at the faulting sites called "silanol nests" for ZSM-5 while this mechanism for ZSM-12 was different. Acidic properties of parent and desilicated zeolites were investigated by the adsorption of pyridine and CO followed by IR spectroscopy. For both zeolitic series the number of Bronsted acid centres was virtually independent of the desilication progress (differences below 20%), while the number of Lewis centres increased by more than 100% in some cases. Pyridine thermodesorption experiment revealed the presence of the second type of Bronsted sites in desilicated zeolites, their acidic properties may be compared to the hydroxyls existing in mesoporous materials of M41S family. (C) 2010 Elsevier B.V. All rights reserved.