The synthesis of diphenylmethane (DPM) from benzene and formalin (a mixture of formaldehyde and water) or paraformaldehyde has been studied using various solid acids. The reaction between benzene and paraformaldehyde (oligomers of formaldehyde) proceeded readily at 140 degreesC on typical solid acids. The activity of a silica-composite of polymer resin, Aciplex-SiO2, was Found to be superior to that of other solid acids such as zeolites (HY,H-ZSM-5 and beta -zeolite), heteropoly compounds (H3PW12O40 and Cs2.5H0.5PW12O40), and the other polymer resins (Nafion-H, Nafion-SiO2, and Amberlyst). The addition of water to the reactant mixture decreased greatly the activity of Aciplex-SiO2. While the reaction between benzene and formalin (27.3 mol% HCHO aqueous solution) was indeed very slow over Aciplex-SiO2 at 140 degreesC, the increase in the temperature to 180 degreesC brought about 42.1% conversion of HCHO with 88.2% of selectivity to DPM at 6 h, when a mixture of 40 cm(3) benzene (450 mmol) and 3.36 cm(3) formalin (HCHO 45 mmol and H2O 111 mmol) and 1 g of Aciplex-SiO2 were used. The turnover number (TON), which was defined as the number of product molecule divided by the number of acid sites in the whole bulk, reached about 36 over Aciplex-SiO2 under the above reaction conditions, indicating a catalytic reaction. This is the first example of catalytic synthesis of diphenylmethane from formalin. The high catalytic performance of Aciplex-SiO2 can probably be attributed to the large number of acid sites (0.46 mmol g(-1)) available for the surface reaction, together with the strong acidity and the high hydrophobicity.