Mixed micelles of sodium dodecyl sulfate (SDS) with Triton X-100 (TX100) or dodecyl octa(ethylene glycol) monoether (C12E8) at different bulk SDS mole fractions (1 and at two ionic strengths were analyzed by frontal analysis continuous capillary electrophoresis. For most systems, the electropherograms showed a distribution of mobilities, providing evidence of compositional heterogeneity in the mixed micelles. From the empirical relationship between average mobility and Y, the mobility distributions may be interpreted in terms of a distribution of micelle compositions. Broad distributions were observed from 0.1 less than or equal to Y less than or equal to 0.6 in all cases, while the mixed micelles were close to monodisperse at Y greater than or equal to 0.8. As similar results were obtained from monodisperse C12E8 and polydisperse TX100, the compositional distributions do not result from the compositional polydispersity of the nonionic surfactant. These results were found to be correlated with the composition dependence of the critical micelle concentration of mixed micelles.