A new method for measuring the surface acidity of solid acid catalysts is described. A Stanton Redcroft combined TG-DSC instrument has been modified to allow the introduction of a series of controlled pulses of ammonia into the helium stream passing over the sample. The weight of ammonia adsorbed by the sample and the enthalpy of adsorption are interpreted in terms of the abundance and strength of the surface acid sites. Surface acidity measurements made on three acid-activated clays and an acidic sulphonated ion-exchange resin are compared with catalytic activity data in two model reactions.