We have investigated a novel approach to well-defined peptide-decorated cross-linked nanoparticles. The particles were obtained from well-defined poly(gamma-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 548 000 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 crosslinker 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.