The catalyst known as "clayzic" (ZnCl2 supported on acid-treated montmorillonite) exhibits both Bronsted and Lewis surface acidities. The relative catalytic activities of these two types of acid site have been determined in a range of reactions, by comparing the activities of clayzic with those of related catalysts which exhibit only one type of surface acidity. Acid-treated montmorillonite has been used as the Bronsted acid catalyst in this comparison, and ZnCl2 supported on a mesoporous silica has been used as the Lewis acid catalyst. The results confirm that the relative activities of the two types of acid site depend on both the nature of the reaction and the polarity of the reaction medium. They will be of value in optimising the choice of support and the ZnCl2 loading in clayzic-type catalysts for specific reactions.