Strong organic acids, CCl3COOH, CHCl2COOH, CF3COOH, CH3SO3H, CF3SO3H, and p-CH3C6H4SO3H, effectively catalyzed the chloromethylation of m-xylene with hydrochloric acid and trioxane under organic and aqueous biphasic conditions. They were active enough at 2-10 wt % against m-xylene to yield chloromethyl-2,4-dimethylbenzene (I) and 1,3-bis(chloromethyl)-4,6-dimethylbenzene (II), although they accompanied trischloromethylation products, trischloromethyl-1,3-dimethylbenzenes (III). The highest yield of II, as high as around 70%, was obtained by using CF3SO3H under appropriate conditions because it has the strongest acid strength among the acids. These acids stay in aqueous phase after the catalysis, and organic products are easily separated from the reaction mixture. The reaction may start the formation of chloromethanol from formaldehyde, and strong organic acids can assist the stabilization of chloromethyl carbocation-acid complex formed by the dehydration of chloromethanol. The resulting complex gives I by nucleophilic attack at o- and p-positions of methyl groups, and II is formed through consecutive attack of the complex against I. The chloromethylation of biphenyl (BP) in the presence of strong acids also gave the chloromethylated products; however, large amounts of acid are necessary for the efficient formation of the products due to the low reactivity of BP.