Three montmorillonites have been acid–activated using a series of acid–clay ratios. The properties of these acidactivated clays [including cation-exchange capacities (CECs), elemental compositions and percentages of remaining oxide] were found to be dependent upon the acid : clay ratio used as well as the nature (including extent of isomorphous substitution and presence of extraneous impurities) of the original montmorillonites. The acid activated clays and the original raw montmorillonites were pillared with alumina to produce pillared acid-activated clays (PAACs) and the corresponding conventional pillared clays (PILCs). The suitability of the acid activated clay to act as a host for the alumina pillars depended on the extent of destruction of the clay as a result of the activation. The properties of the PAACs were found to be different from those of the conventional PILCs. The PAACs incorporated less AI than did the conventional PILCs and the average pore diameters were higher, the PAACs being more mesoporous and less microporous than the conventional PILCs. The total pore volumes of the PAACs were higher and the surface areas increased with acid : clay ratio to a maximum before decreasing. The PAACs possessed enhanced catalytic ability (up to 60%) for the dehydration of pentanol at 200 °C compared to the PILCs, owing to increased surface acidity. The nature of the acidity of the PAACs was found to depend upon the identity of the original clay and it appears that at any one acid : clay ratio, a balance exists between the sources and strengths of the acid sites.