Hydroxyl groups located on the external surface and in the pore mouth region of HZSM-5 zeolites with different crystal sizes were modified by chemical liquid deposition (CLD) of tetraethoxysilane (TEOS). The silylation of the acid sites was monitored using NH3 sorption, IR spectroscopy of the zeolite hydroxyl groups, and adsorption of pyridine and di-tert-butyl-pyridine (DTBPy). Silylation reduced the concentration of Bronsted acid sites, preferentially in the pore mouth region of the zeolites, and led to a blocking of Lewis acid sites. The deactivation of Lewis acid sites was found to occur prior to the reaction with bridging hydroxyl groups and silanol groups. The efficiency of silylation achieved was found to depend on the particle size of the zeolite sample. A multicycle procedure can be used to increase the effects of the modification with TEOS.