In order to investigate the role of methanol-decomposition in the side-chain alkylation of toluene with methanol, methanol-decomposition was carried out on alkali-ion exchanged zeolites X with or without boron. Yield of formaldehyde which has been known as a true alkylating reagent in the side-chain alkylation was decreased with increase of radius of cation exchanged, but seemed to be independent on the presence of boron. It was characteristic that boron led to suppress the further decomposition of formaldehyde to carbon monoxide. According to these results, it was suggested that the increase of styrene-yield on the alkali-ion exchanged zeolites X was mainly influenced by the adsorption configuration of toluene rather than the yield of formaldehyde.