The effect of 18-crown-6 (18C6) on the decomposition of N-methyl-N-nitroso-p-toluenesulfonamide (MNTS) by N-3(-) and H3O+ in micelles of tetradecyltrimethylammonium azide (TTAN(3)) and hydrogen dodecyl sulfate (HDS) has been studied. The crown ether solubilizes in TTAN(3) micelles only in its uncomplexed form due to hydrophobic effects because the positively charged TTAN(3) micellar surface avoids incorporation of 18C6 complexed with cations. In HDS, besides hydrophobic effects, 18C6 binds to the micelles by complexing H3O+ counterions. The hydronium complexation constant of 18C6 in the micellar phase is much larger than its corresponding value in water, which was explained on the basis of changes in the microenvironment of the reagents at the water-micelle interface. The results obtained allowed us to discuss the different locations of the micelle-bound crown ether both in its free and complexed forms : the uncomplexed crown ether is located in an inner region of the micelle, whereas 18C6 . H3O+ is preferentially located in the Stern layer acting like a counterion and modifying the bimolecular rate constant of MNTS decomposition in the micellar phase.