A systematic study of the effect of acid washing of sodium mordenite (Na-MOR) and hydrogen-exchanged mordenite (H-MOR), both before and after steaming, has been carried out. The study investigated the formation and removal of extra-framework aluminium and the effect of dealumination on the acidity and isobutane cracking activity of the mordenite. Washing ammonia-exchanged mordenite (NH4-MOR), which has been calcined at 550°C, with deionised water resulted in an increase in extra-framework octahedral aluminium (EFAl-o). The corresponding decrease in tetrahedral aluminium and the sharp drop in catalytic activity is ascribed to the hydration of strong acid sites. Both acid washing and thermal treatment of Na- and NH4-MOR produced EFAl-o. A systematic study of the washing of Na- and H-MOR with nitric acid of increasing concentrations showed that a maximum occurrence of EFAl-o was reached by washing with 0.001 N acid in the case of H-MOR. Thereafter EFAl was removed with little accompanying framework dealumination up to an acid wash concentration of 1 N. Further washing up to 10 N removed large amounts of framework aluminium. A maximum in catalytic activity for isobutane conversion was observed for a sample washed with 1 N acid. Generally, however, catalytic activity increased with the number of framework acid sites. Extra-framework tetrahedral aluminium (EFAl-t) and penta-coordinated aluminium (Al-p), both produced by steaming, reduced both the catalytic activity and the number of framework acid sites. These aluminium species were progressively removed by increasing strength of acid washes. The number of Brönsted and strong Lewis acid sites as determined from ammonia temperature-programmed desorption (TPD) were found to be essentially equal to the amount of framework aluminium not charge-balanced by non-framework cations such as sodium and tetrahedral extra-framework aluminium.