Microcalorimetry was used to determine the differential enthalpy changes of adsorption versus coverage on H-ZSM-5 and H-mordenite of reactants and products of methylamine synthesis. The enthalpy changes of adsorption of dimethylether, ammonia, monomethylamine, and dimethylamine on Bronsted acid sites vary linearly from -90 to -250 kJ/mol with the gaseous proton affinities of these basic molecules. The enthalpy changes of adsorption of water and methanol vary from -60 to -90 kJ/mol. In situ infrared spectroscopy indicates that methoxyl species are present on the catalyst in flowing methanol at temperatures near 600 K; however, adsorbed ammonia and methylamines are associated with the Bronsted acid sites in gas mixtures of methanol and ammonia typically used for methylamine synthesis at these temperatures. These results suggest that dimethylether formation from methanol may occur through surface methoxyl species, while methylamine synthesis most likely involves adsorbed ammonium cations.