It is commonly assumed that adsorbed basic probe molecules freely equilibrate with surface-acid sites in microcalorimetric experiments to determine the strength and distribution of the acid sites. The validity of this assumption was tested by comparing the differential heat of adsorption on H-USY as a function of surface coverage of CD3NH2, NH3 and CD3CN, which have widely different proton affinities, and by monitoring with FTIR the distribution of the adsorbed molecules between Bronsted-and Lewis-acid sites during adsorption and desorption. The results showed that full equilibration was achieved with the weakest base, CD3CN, but not with the stronger bases, CD3NH2 or NH3.