We have investigated a novel approach to well-defined peptide-decorated cross-linked nanoparticles. The particles were obtained from well-defined poly(¿-benzyl-l-glutamate-b-styrene) block copolymers with active nitroxide end-groups by reaction with divinylbenzene (DVB). The block length ratio and the amount of cross-linker (DVB) were systematically varied. Molecular weights of the cross-linked particles up to 548000 g/mol with polydispersities around 1.5 were confirmed by size-exclusion chromatography. A clear dependence of the molecular weight from the block length ratio and the amount of cross-linker was observed. This was confirmed by dynamic light scattering results revealing the formation of nanoparticles up to 21 nm in size. The core-shell structure was evident from the TEM micrographs. Further deprotection of the peptide shell yielded water-soluble pH-responsive nanoparticles with a poly(l-glutamic acid) shell and a polystyrene core. Cryo-TEM micrographs confirmed the presence of individual core-shell particles in the aqueous solution.