Hybrid polymeric micelles with compact polystyrene cores (PS) and mixed poly(methacrylic acid)/poly(ethylene oxide) shells (PMA/PEO) were prepared by mixing PS-PMA and PS-PEO linear diblock copolymer samples in l,l-dioxane (80 vol %)/water. The mixture is a mild selective precipitant for PS and micelles with swollen PS cores, and either PMA or PEO shells form in both systems before mixing. Both types of micelles coexist in a mobile equilibrium with unimers and, because PMA and PEO are fairly compatible, hybrid micelles PS-(PMA/PEO) form spontaneously after mixing the individual micellar solutions. Hybrid micelles differing in the composition of the mixed shell were stepwise dialyzed into aqueous buffers. In aqueous media, the unimer-micelle equilibrium is kinetically frozen and the micelles behave as independent polymeric nanoparticles with glassy PS cores and mixed polyelectrolyte/nonelectrolyte shells. A number of experimental techniques (size-exclusion chromatography, capillary zone electrophoresis, and elastic and quasielastic light scattering measurements) were applied for characterization of the hybrid micellar systems.