Magic angle spinning (MAS) NMR spectroscopy of N-15 has been used for the quantitative analysis of sterically unhindered acid sites of three acid zeolites, mordenite (HMOR), beta (HB). and ultrastable Y (HUSY), using as base di-tert-butylpyridine (DTBP) labeled with N-15. Separate signals for protonated (DTBPH+) and for the nonprotonated (excess) DTBP were observed, indicating that the proton transfers involving DTBP, DTBPH+, and the acid sites are slow on the NMR time scale. The DTBPH+ ions are rigid on the surface, whereas the nonprotonated molecules are mobile and observable by high-resolution ("liquid-state") NMR. The integration of the N-15 spectra indicates that about 31%, 36%, and 90% of the acid sites (aluminate structural units in the framework) can transfer protons to DTBP in HUSY, HMOR, and HB, respectively. In the C-13 spectra of the cations, the ring carbons gave broad signals, which cannot be made narrower by MAS, suggesting that the cations are frozen on the solid in positions differing in chemical environment. The changes in the chemical shifts of the a ring carbons and of the aliphatic carbons indicate that some distortion of the bonds connecting the tert-butyl groups with the ring probably exists for both the protonated and nonprotonated DTBP in the cavities, even though the latter molecules enjoy enough freedom of movement to afford spectra under high-resolution conditions. The distortion indicates that both the chemisorbed and the physisorbed molecules are present mostly in the channels, rather than on the external surface of the crystals. A site can be inaccessible either because it is sterically hindered inside the zeolite structure, or because it is so close to another site that two molecules of base cannot fit to form tight ion pairs with the anions, which we found a prerequisite of protonation. The NMR method, made possible by the use of labeled DTBP. is simpler and more accurate than the IR measurement. It should be very useful in the Studies of acid sites of mesoporous materials.