Rare-earth metal triflates, such as Sc(OTf)₃, Yb(OTf)₃, and Sm(OTf)₃ work as highly effective catalysts for the chloromethylation of aromatic hydrocarbons with hydrochloric acid and trioxane. They are active enough in aqueous solution at a concentration of less than 1–5% of the substrate under heterogeneous conditions of organic and aqueous phases. The triflate stays in the aqueous phase after the catalysis, and the organic products are easily separated from the catalyst. Sc(OTf)₃ was stable under the reaction conditions. The catalyst in the aqueous solution could be recycled and used for further reactions without significant loss of activity. The chloromethylation of m-xylene catalyzed by Sc(OTf)₃ gave 2,5-bis(chloromethyl)-m-xylene with 1,2,4,5-substitution, which is expected to be a key intermediate for pyromellitic dianhydride. The catalysis occurred via the formation of a chloromethylated triflate complex, and electrophilic addition to an aromatic hydrocarbon.