Two commercially available acid-activated montmorillonite clay catalysts, K5 and K10, have been characterised by ammonia adsorption microcalorimetry at 150 degreesC. The adsorption capacities and molar enthalpies of adsorption have been interpreted in terms of the concentration and strength of surface acid sites. Clays exchanged with Al3+, Fe3+ and Na+ have been examined. Catalytic activities have also been measured, in liquid phase reactions in swelling and non-swelling solvents, and in reactions catalysed by Bronsted and by Lewis acid sites. The relationships between ammonia adsorption characteristics and catalytic activities have been determined. It has been shown that anhydrous ammonia is able to access all acid sites on the clay surface, including those in the residual clay interlayer regions which would normally only be accessible in the presence of a swelling solvent. Ammonia adsorption on clays exchanged with Fe3+ where Lewis acidity is maximised, is more extensive and with a higher molar enthalpy of adsorption than on clays maximised for Bronsted acidity. It is concluded that ammonia can be satisfactorily used as a probe of surface acidity when Bronsted acidity dominates but, if there is significant Lewis acidity, ammonia adsorption data is difficult to interpret. (C) 2003 Elsevier B.V. All rights reserved.