Intramicellar crosslinking of the polymer chains within the shells of polystyrene-b-poly(acrylic acid) micelles by reaction with difunctional poly(ethylene oxide) afforded unimolecular amphiphilic core-shell nanospheres (50 nm hydrodynamic radius); the resulting surface hydrogel layer gives an approximate fivefold increase of particle volume from the dry state to aqueous solution.