In situ infrared spectroscopy was used to study sorption and reaction of toluene and methanol over various basic catalysts (MgO, hydrotalcites, and basic zeolites). The size of the metal cations controls the preference of sorbing methanol or toluene; i.e., the larger the metal cation, the higher the preference for toluene. The key requirements for a good catalyst for side-chain alkylation are (i) sufficient base strength to dehydrogenate methanol to formaldehyde, (ii) stabilization of sorbed toluene and polarization of its methyl group, and (iii) balanced sorption stoichiometry of the two reactants. The formation of the carbon-carbon bond mechanistically resembles an aldol condensation.