K-10 montmorillonite, commonly used as a heterogeneous acid catalyst, was found to vary in the extent of acid-treatment, with some batches exhibiting significantly reduced catalytic activity in Bronsted acid-catalysed reactions. K-10 was thus further treated with HCl of varying concentrations to increase its activity in acid-catalysed reactions. Acid-treated clays exhibited significant enhancements in catalytic activity in three test reactions; tetrahydropyranylation of ethanol, diacetylation of benzaldehyde and esterification of succinic anhydride. Acid-treatment of K-10 was shown to result in protonation, and loss of layer stacking of the clay structure, as determined by powder X-ray diffraction, inductively coupled plasma–optical emission spectroscopy (ICP-OES), diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and Brunauer–Emmett–Teller (BET) specific surface area measurements. Quantifiable physical changes to the K-10 correlated with measurable increases in catalytic activity. Standard procedures for assessing acid-treated montmorillonite clay catalysts, such as K-10, and procedures for obtaining the most effective catalyst for acid-catalysed reactions, involving analytical and synthetic techniques, were devised.