Distribution of the acid-base strengths of various ion-exchanged montmorillonites was assessed through thermal programmed desorption (TPD) of NH3 and CO2. Accurate acid-base measurements can be achieved via deconvolution of perfectly symmetrical peaks, under optimal carrier gas throughput and heating rate, estimated though a factorial experiment design. No neutral clay samples without interactions with carbon dioxide or ammonia were found. All clay samples displayed both acid and base characters, with specific TPD patterns, and no pure acidic or pure basic clay materials were obtained. Increasing cation radius led to increasing acidity and decreasing basicity, in terms of both total amount of desorbed probe gas and strength of the adsorption sites. It was suggested that basicity and weak acidity are intrinsic features of the clay structure, while medium and strong acidities are induced by the exchangeable cation. (c) 2006 Elsevier B.V. All rights reserved.