The ultrasonic relaxation associated with the perturbation of the monomer/micelle exchange process in sodium dodecyl sulfate and sodium dihexylsulfosuccinate in the presence of 10 -4 (mol dm -3) NaBr has been studied. Membrane electrodes selective to both surfactants were constructed and their emf measured relative to both a commercial bromide and a sodium electrode. From these emf data the monomer concentrations of both surfactants in the micellar range and also the degree of micellar dissociation were measured. The kinetic parameters associated with the monomer micelle exchange process and the polydispersity of the micelles have been evaluated using different versions of the relaxation equation originally derived by Aniansson and Wall and also using a phenomenological treatment. Finally, the volume change during micellization and the polydispersity of the micelles were evaluated from the concentration dependence of the relaxation amplitude.