Synthesis of diphenylmethane (PhCH2Ph) from benzene and formalin (aqueous HCHO) was investigated in a biphasic reaction system, which consisted of benzene and aqueous phases, where the latter phase contains HCHO and liquid acid as catalyst. Heteropolyacids such as H3PW12O40, H3PMo12O40, and H4SiW12O40 exhibited significant activities for the formation of diphenylmethane, whereas typical mineral acids (H2SO4, HNO3, and H3PO4), organic acids (para-toluenesulfonic acid (PTS) and CF3COOH), and Sc(CF3SO3)(3) were inactive. The yields of diphenylmethane were proportional to the concentrations of both proton and the heteropoly anion (PW12O403-). Consequently, it can be presumed that the critical reaction step is the attack by a benzyl cation to a benzene molecule, in which the intermediate benzyl cation is stabilized by a heteropoly anion to accelerate the reaction. Since heteropolyacids exist in the aqueous phase exclusively, and the product diphenylmethane is concentrated in the benzene phase after the reaction, H3PW12O40 can be readily recovered from the reaction solution. It was confirmed that H3PW12O40 in the aqueous phase, after drying at room temperature, could be reused repeatedly without significant decreases in the activity. (C) 2003 Elsevier B.V. All rights reserved.