In situinfrared spectroscopy was used to study the sorption and reaction of toluene and methanol over various alkali exchanged X zeolites. The size of the metal cations controls the preference of sorbing methanol or toluene. The smaller the cation, the higher the preference for methanol is. A balanced sorption stoichiometry of the reactants, the intermediate conversion of methanol into formaldehyde and the strong polarization of the methyl group of toluene are critical to catalyze side chain alkylation of toluene. These requirements are best met with very basic zeolites such as the Cs-X and Rb-X.