The catalytic activity of acid-treated montmorillonite clay towards Bronsted acid catalysed reactions is shown to be highly dependent on the degree of acid treatment received by the clay. Using two contrasting model reactions, one involving highly polar reactants and the other a non-polar reactant, the optimum treatment conditions for an acid-treated clay catalyst are also shown to depend upon the type of reaction being catalysed. In addition, the role of exchangeable Al3+ ions in generating catalytically active acid sites in the two reactions is investigated.