The acidic properties of dealuminated Y-type zeolites were characterized by infrared (IR) spectroscopy, microcalorimetry, 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy and temperature-programmed desorption (TPD). Microcalorimetric measurements exhibited a uniform heat of adsorption (140 kJ/mol) of ammonia on the strong Brönsted acid sites. The differences in the acid site concentrations measured by adsorption of ammonia from the gas phase and by decomposition of ammonium-exchanged zeolites are discussed. The results indicate that parts of the extra lattice material consisting of cationic aluminium oxide species or silica alumina species are removed by ion exchange with aqueous solutions of ammonium hydroxide. Based on this, a method for the controlled removal of extra lattice material was developed.