A set of ultrastable Y zeolites with different amounts of framework and nonframework aluminum was prepared by fluorination of H-USY zeolite. The samples were characterized by chemical analysis, physical adsorption of nitrogen at low temperature and of pentane at room temperature, XRD, Si-29, and Al-27 MAS NMR. n-Pentane isomerization has been studied mainly at 285 degrees C, while the activation energy was measured on several samples. The fluorine treatment removed Al from the framework position and stabilized the zeolite structure. Hence, it decreases the number of Bronsted sites. Despite an increase in the amount of nonframework aluminum, the number of Lewis sites decreases; this means that the dispersion of the Lewis sites on the nonframework alumina particles decreases dramatically. The catalytic activity appears to be a direct function of both the number of Lewis and the number of Bronsted acid sites. These numbers being close to one another, it might be that the "acid site" is, in fact, a complex resulting from the interaction between the proton donor and an electron acceptor.