The acid-base properties of a series of cationic faujasite zeolites have been studied using methylamine as a probe molecule by means of in situ infrared spectroscopy. The results have been correlated with the nature of counterions and the Si/Al ratio of the studied zeolites. The present work reveals three kinds of interaction between methylamine and cationic zeolites. Methylamine can interact not only with alkali cations via the lone electron pair on nitrogen atoms but also with the negatively charged oxygen atoms of the framework via the hydrogen atoms of both NH2 and CH3 groups as well. The interaction between the methyl groups and these oxygen atoms is for the first time postulated. However, the Lewis acidity of counterions remains the dominating factor in the interaction between methylamine and cationic zeolites. A deformation of the zeolite framework due to the adsorption of methylamine has been observed. This work shows clearly that methylamine can be an efficient probe molecule for the characterization of acid-base properties of zeolites.