For the first time, EPR spectra have been obtained which unambiguously distinguish between paramagnetic surfactants in monomeric and micellar environments. The frequency with which the paramagnetic surfactant 2,2,6,6- tetramethylpiperidine-oxidedodecyldimethylammonium bromide exchanges between micellar and aqueous environments increases from about 105 s–1 at 24°C to 1.7 × 106 s–1 at 68°C. From these data an enthalpy of activation for the micellization process of approximately 9.5 kcal/mol has been determined.