Variable-temperature C-13 NMR and N-15 NMR with magic angle spinning were used to characterize the formation of imines by the reactions of benzaldehyde or acetone with ammonia on zeolites. When ammonia was adsorbed at cryogenic temperatures on zeolite HY (FAU) which was preloaded with benzaldehyde, it formed an appreciable amount of the tetrahedral gem-amino-hydroxy intermediate at temperatures as low as 163 K. As the sample was heated to slightly above room temperature; the intermediate dehydrated to form imine. The intermediate is persistent at 298 K and yields distinct isotropic chemical shifts at 78 ppm for C-13 and -320 ppm for N-15. Benzaldehyde was cleanly converted to imine by reaction with ammonia on the medium-pore zeolite HZSM-5 (MFI), but no gem-amino-hydroxy intermediate was seen. We attribute this effect to the steric and reactivity differences that favor dehydration to form the less bulky imine product on HZSM-5. The experience of solution chemistry suggests that gem-amino-hydroxy intermediates from ketones are less stable than those from aldehydes, and this was also confirmed for reactions on zeolites. However, a small C-13 Signal at 81 ppm was observed during the low-temperature reaction of acetone and ammonia on HY, and this shift is identical to that seen for 2-aminopropan-2-ol in liquid ammonia. Low loadings of acetone and ammonia on HZSM-5 formed the imine cleanly, and this was stable with little secondary chemistry to 523 K. Secondary reactions were more pronounced for acetone and ammonia on HY, and these were used to explore and compare several reactant adsorption protocols. Sample heating generated a sequence of nitrogen-substituted products analogous to those seen in aldol reactions of acetone on the same zeolite. These species were partially converted to aromatic hydrocarbons at 523 K, but no pyridines were formed with these reaction conditions. The imines showed evidence of protonation-and hydrogen bonding to the conjugate base site of the zeolite, We also characterized the temperature-dependent dynamics of imine products that exhibited broad C-13 signals at room temperature, but sharp lines at low and high temperatures. The results of this study were used in combination with published theoretical studies of the interaction of ammonia with zeolites to suggest models of the association of reactants, intermediates, and products with the framework site.