The acidity of a series of commercial dt aluminated H-Y zeolites was studied by means of MAS NMR and XPS spectroscopy. Al-27 MAS NMR has shown the presence of three types of Al in the dealuminated samples: framework tetrahedral Al, nonframework octahedral Al, and a third type of Al. As inferred from the framework Si/Al ratio obtained by Si-29 MAS NMR spectra and from the evolution of the. unit cell parameter determined by XRD, this third type of Al may be localized into the zeolite framework. XPS characterization of the samples in their acid form has allowed us to establish the relative concentration of Al at the outer surface of the particles. Upon adsorption of ammonia on the dealuminated zeolites, three types of acid sites have been evidenced by XPS. The analysis of the XPS data indicates that the strength of the acid sites associated with tetrahedral framework Al atoms is not influenced by the other types of Al. The catalytic results of heptane and decane hydroisomerization have evidenced that the acid sites of certain dealuminated H-Y zeolites may be very efficient. The information provided by MAS NMR, combined with the XPS results and literature data, has allowed us to partially elucidate the origin of the high activity of some dealuminated H-Y zeolites.