Aqueous solutions of poly(vinyl acetate)-b-sodium polyacrylate (PVAc-b-NaPAA) block copolymers were characterized by capillary electrophoresis (CE). CE experiments reveal the presence of NaPAA dead chains, acetate ions (which are a byproduct of the hydrolysis of PVAc), and nonassociated copolymers (unimers). This fraction of unimers is dependent on the chemical composition of the copolymers and on the purification procedure. It remains constant when increasing copolymer concentration. The fraction of unimers was too high to correspond to a critical micellar concentration. Instead, this high content of unimers is likely due to the polydispersity in composition of the copolymers, sodium polyacrylate-rich copolymers being excluded from the micellization process. This interpretation is consistent with the study of the effect of temperature on the free copolymer fraction. Above a temperature threshold, the unimer population increases due to the destabilization of part of the micelles. Addition of a neutral surfactant leads to the formation of mixed micelles, the electrophoretic mobility of which can be modeled using recent theoretical developments on electrokinetic migration of composite objects.