Adsorption isotherms of dodecyltrimethylammonium bromide (DTAB) and decyltrimethylammonium bromide (DeTAB) on mordenites of different Si/Al ratios were determined in the absence and presence of sodium chloride. A surfactant ion-selective electrode was used for determining the equilibrium concentration of unbound surfactant ions. DTAB showed a stronger adsorption than DeTAB did and both surfactants interacted more strongly with mordenites of higher Si/Al ratio. The data analysis by a proposed modified BET treatment, which included site adsorption followed by an aggregation process on the site already occupied by surfactant ions, indicated that the adsorption constant for the site adsorption increased as the Si/Al ratio increased and the free energy change for the adsorption of DeTA(+) ion was reduced by 3.1 kJ mol(-1) for each increase in the Si/Al ratio of mordenites. These observations were ascribed to the increasing hydrophobic interaction between surfactant and mordenite with increasing Si/Al ratio.