Methylamine synthesis from methanol and ammonia over parent and modified Bronsted acidic mordenites is studied by in situ infrared spectroscopy and kinetic analysis to elucidate the role of elementary steps for activity and selectivity. lit situ infrared spectroscopy reveals that all methylammonium ions are formed in the micropores of these catalysts. The formation of the chemisorbed methylamines, however, is not rate determining. Transient response experiments indicate that the desorption of these methylamines aided by adsorbing ammonia and/or the scavenging of methyl groups with ammonia constitutes the rate determining step. For a given catalyst, the selectivity strongly depends on the methanol conversion and the ammonia to methanol ratio of the feed. Upon modification of mordenite with tetraethoxysilane, the selectivity to the lower methylated amines is strongly enhanced. The transport limitations of the bulkier products, formed in high concentration in the pores, are concluded to cause the enhanced selectivity toward mono- and dimethylamine over the modified catalyst. Since a decrease in activity compared to the parent sample was not observed, it seems that the methyl-scavenging mechanism plays an important role over these catalysts.