The acid properties of H-ZSM-5 zeolites with different SiO2/Al2O3 ratios (30, 50, 80, 150, 280 and 1000) were examined by means of the temperature programmed desorption of ammonia (NH3-TPD). Different pretreatments together with a comparison of different curve-fitting methods after desorption of ammonia were used to differentiate between different adsorption sites. 27 Al-MAS-NMR spectroscopy was applied to distinguish the framework (AlF) from the extra-framework aluminum (AIEF). Both techniques reveal that the concentration of Bronsted acid sites is lower than expected when considering the aluminum content. The strength of the Bronsted acid sites, determined by a theoretical model assuming the free readsorption of ammonia, is found to be in the range of Delta H = 129-161 U mol(-1) and shows variations in the acid strength to be less than a = 12 U mol(-1). (c) 2007 Elsevier B.V. All rights reserved.