We present a first experimental observation and provide a theoretical interpretation of a pH-induced micelle-to-micelle phase transition in aqueous solutions of spherical block copolymer micelles with polybasic coronas. Dynamic light scattering, static light scattering, and atomic force microscopy confirm sharp changes in micellar hydrodynamic size and aggregation number occurring in a narrow pH range, DeltapH<0.1. In agreement with theory, zeta potential measurements indicated an abrupt change in ionization of polymer chains in the micellar corona at the transition pH.
