The acidic and catalytic properties of sulfonic acids supported on polystyrene, on silica (via propyl and phenyl tethers) and on a fluorinated hydrocarbon polymer (Nafion) are compared. Surface acidities are characterised using ammonia adsorption calorimetry under flow conditions in which pulses of ammonia are introduced to the sample from a flowing carrier stream. The extent of adsorption and molar enthalpies of ammonia adsorption (Delta H-ads degrees) are interpreted in terms of the abundance, accessibility and strength of surface acid sites. Catalytic activities are measured for the isomerisation of alpha-pinene. The Nafion catalysts show the highest Delta H-ads degrees(NH3) and the highest catalytic activities. Although both silica-supported and polystyrene-supported sulfonic acids show lower specific activities and lower Delta H-ads degrees(NH3) values, the correlation between activity and Delta H-ads degrees(NH3) is relatively poor for these supported forms of the acid. It appears that while Delta H-ads degrees(NH3) is certainly sensitive to the strength of acid groups on which ammonia is adsorbed, it can only be used to compare acid strengths in a meaningful way for structurally similar catalysts. (C) 2006 Elsevier B.V. All rights reserved.