The structure of charged spherical block copolymer micelles in aqueous solution was investigated with static and dynamic light scattering, small-angle neutron scattering, and cryo-electron microscopy as a function of added salt. At low added salt concentration the polyelectrolyte shell has typical features of an osmotic brush. If the added salt concentration exceeds the intrinsic ionic strength of the polyelectrolyte shell, the micellar aggregation number increases due to screening of the repulsive interaction between polyelectrolyte chains (salted brush). The relation between shell ionic strength and added salt concentration follows a simple Donnan equilibrium. The combination of methods reveals that the polyelectrolyte shell is phase-separated into a dense interior and a dilute outer domain.