C-13 NMR spectroscopy has been used to characterize the adsorption of acetone (C-2 C-13-labeled 2-propanone), at coverages of less than one molecule per Bronsted-acid site, in the molecular sieves H-ZSM-5, H-mordenite, H-ZSM-12, H-ZSM-22, chemically dealuminated H-Y, SAPO-5, MgAPO-5, and BeAPO-5. It is shown that the trace of the chemical shift tensor for the carbonyl carbon at 125 K measures the ability of the site to form a hydrogen bond with the carbonyl oxygen and, therefore, can be used as a measure of the intrinsic acidity of the site. The sites in all of the materials investigated were nearly homogeneous, with the isotropic shifts for most of the molecular sieves falling in the range observed for solutions of acetone in trifluoroacetic acid. In addition, the lineshape and linewidths of the C-13 NMR powder patterns at room temperature provide a measure of the molecular motion of adsorbed acetone and are strongly affected by the size of the framework channels.