Calorimetric pulsed NH3 adsorption measurements from a flowing carrier gas have been made on sulfonated poly(styrene-co-divinylbenzene) resin acid catalysts to characterise surface acidity in general and acid site accessibility in particular. The adsorption and enthalpy profiles associated with each pulse of NH3 show distinct features associated with relatively weak and reversible adsorption of NH3 followed by strongly exothermic reaction with sulfonic acid sites and irreversible adsorption. The rate at which NH3 diffuses from its initial adsorption site to an acid site is shown to depend on catalyst particle size, adsorption temperature and the proportion of unreacted acid sites. By varying the contact time of NH3 with the catalyst the reversibility of the initial NH3 adsorption is demonstrated and an increasing number of the acid sites within the polymer matrix are shown to become completely inaccessible to NH3 as the contact time is reduced. (c) 2006 Elsevier B.V All rights reserved.