Capillary electrophoresis (CE) was used to study the polydispersity of TX-100/SDS and C(12)E(8)/SDS micelle systems. Compared with other techniques, CE provides more specific and graphically detailed information on the compositional polydispersity of such micellar systems. Clear evidence of the coexistence of micelles rich in either nonionic or anionic component has been obtained. This coexistence is explained by considering that the stability of mixed micelles is affected by several factors, including counterion binding, and steric shielding of the electrostatic repulsive interaction among the headgroups of SDS by the bulky EO groups of TX-100. Due to these two factors, micelles with different compositions may be energetically equivalent, leading to a broad distribution of micelle size and composition. In this study, we observe how the pattern of the distribution changes with the mole fraction of SDS. Electrophoretographic patterns also showed that broader compositional distribution of the original components produces a wider distribution of mobilities.