A flow microcalorimetric technique for characterising the acidities of sulphonated polystyrene resins by ammonia adsorption from the gas phase has been applied to a series of macroporous and gel-type resin catalysts. The relationship between acidities measured using this technique and catalytic activities towards liquid phase reactions has been examined using two acid-catalysed test reactions. Results show that catalytic activities are not related in a simple way to the concentrations of acid sites measured by ammonia adsorption, but there is a useful correlation between the strengths of the acid sites measured in this way and catalytic activities of the resins. We conclude that acid site strength, based on ammonia adsorption and used alone, can be a useful indicator of activity for this type of acid catalyst.