The acid and alkaline hydrolyses of N-methyl-N-nitroso-p-toluenesulfonamide (MNTS) in the presence of cationic vesicles of the surfactant dioctadecyldimethylammonium chloride (DODAC) were studied. The ion-exchange pseudophase model adapted to vesicle systems was used to fit the experimental results. An association constant for MNTS with the vesicles of 380 +/- 30 M-1 was determined by kinetic and spectrophotometric methods. The rate constant for the alkaline hydrolysis of MNTS in the vesicular pseudophase was also determined, and its value (0.0084 M-1 s(-1)) was found to be about one-tenth that in water and slightly smaller than that in CTABr or TTABr micelles. This intrinsically decreased reactivity is related to the different polarities in these three media.