The adsorption of the alkali-metal cation (M=Li + , Na + , K + , Rb + , and Cs + ) and the reaction mechanism of the side-chain alkylation of toluene with formaldehyde over MY basic zeolite (M=Rb + and Cs + ) were investigated using density functional theory calculations. During co-adsorption, the zeolite adsorbs toluene preferentially and then formaldehyde. The side-chain alkylation in MY zeolite (M=Rb + and Cs + ) consists of the following two steps: toluene reacts with formaldehyde to obtain phenyl ethanol, and then the intramolecular dehydration of phenyl ethanol produces styrene and water. Phenethyl alcohol dehydroxylation is the rate-controlling step. Although the activation energy is similar in Rb + - and Cs + -modified zeolite (63.09 and 62.20kcalmol −1 ), the products are much easily removed from the system in Cs + -modified zeolite. Scilicet, Cs + -modified zeolite is a suitable catalyst during the side-chain alkylation of toluene.