The dimerization of alpha-methylstyrene was examined at 50 degrees C and 1 atm in nonpolar cumene and polar p-cresol solvents, using both soluble liquid acids and resin-based solid acids as catalysts, In particular, the activities of a representative macroporous sulfonic acid resin, Amberlyst-15(R), and Nafion(R) perfluorosulfonic acid resin in three different microstructures, gel-type Nafion(R) NR-50 resin, a carbon-supported Nafion*(R) resin, and a Nafion(R) resin/silica composite material, have been compared. The Nafion(R) resin/silica composites showed, by far, the highest activity. Supported Nafion(R) resin on carbon and Amberlyst-15(R) are effective catalysts as well and are all more active than pure Nafion(R) resin based on catalyst weight. None of the small molecule acids were sufficiently soluble to show significant catalytic action in the nonpolar solvent cumene. On the other hand, in the polar p-cresol media, the homogeneous acids showed relative activities in the same order as their pK(a)s; and the strongest aqueous acid, triflic acid was more active than any of the resin catalysts on an acid equivalent basis. Among the resin catalysts, only the Nafion(R) NR50 gel showed significant activity enhancement in p-cresol. In order to gain insight into these relative catalytic activities, the acid capacity and accessibility of acid groups in these solid acid catalysts were studied using temperature programmed desorption/thermogravimetric analysis (TPD/TGA) techniques with isopropanol as probe molecule. The TPD/TGA studies show that the sulfonic acid groups are readily accessible to 2-propanol vapor in the 13 wt% Nafion(R) resin/SiO2 composite but not in gel-type Nafion(R) resin and Amberlyst-15(R).