An effective design of solid acid catalysts for synthesis of bisphenol-A was investigated using organic-inorganic dual modification of heteropolyacids. Among various solid acids, partially Cs-ion exchanged tungstophosphate (Cs2.5H0.5PW12O40) showed the highest turnover frequency, i.e., the initial reaction rate per acid site. The acid strengths estimated by differential heat of ammonia adsorption showed that relatively moderate acid strength is optimum for the intrinsic activity of acid sites. Cs2.5H0.5PW12O40 was further modified by immobilization of mercapto compounds. The addition of 2-diethylamino-ethanethiol (DEAT) to Cs2.5H0.5PW12O40 significantly enhanced both activity and selectivity. When the ratio of DEAT and protonic acid site was unity, the designed catalyst showed comparable bisphenol-A yield and higher turnover number and frequency than those of a conventional ion-exchange resin. The high catalytic activity of the dual modified heteropolyacid is attributed to the microscopically designed active center composed of inorganic protonic acid coupled with immobilized organic thiol group. (C) 2010 Elsevier B.V. All rights reserved.