The "degree of ionization" of a micelle or the "apparent degree of counterion dissociation", alpha, is defined by asserting that the aggregation number, N, is dependent only on the concentration, C-aq, of counterions in the aqueous pseudophase. Using different combinations of surfactant and added salt concentrations yielding the same value of C-aq leads to a straightforward definition of alpha. Any experimental method able to precisely distinguish values of N may be used and should give the same values of alpha. The value of N is not needed in the method; the experiment needs only ensure that the value of N is the same for two samples. For systems in which N is a known function of C-aq, alpha may be determined as a function of N. The method is demonstrated with sodium dodecyl sulfate (SDS) using an electron paramagnetic resonance method. For SDS, the method yields a constant value of alpha = 0.272 +/- 0.017 for values of the aggregation number less than about N approximate to 110, which corresponds to combinations of surfactant and salt concentrations from [SDS] = 600 mM and [NaCl] = 0 to [SDS] = 25 mM and [NaCl] = 155 mM. This value of cc is in excellent agreement with literature values based on activity measurements, micelle mobility measurements, and radioisotope mobility measurements of the Na+ ion but in poor agreement with measurements employing light scattering. The constancy of alpha is in accord with theoretical arguments based on the Poisson-Boltzmann equation.