A membrane potential technique has been used to determine the degree of dissociation of eight different surfactant micelles. For two compounds, n-dodecyltrimethylammonium bromide and n-dodecylpyridinium bromide, the measurements have been extended over a 50° temperature range and values of 2.0 and 1.9 kcal mole–1 respectively have been obtained for the energy of desorption of counterions from the micelle surfaces. Estimates of the heat of micellization as a function of temperature are also reported for these two compounds.