Three-layer nanoparticles were prepared by gamma -radiation-induced polymerization of methyl methacrylate (MMA) in polystyrene-block-poly(methaerylic acid) aqueous micellar solution, and their structure was studied by small-angle neutron scattering (SANS). The scattering data were fitted using the bare-core approximation for a two-component core to elucidate the distribution of MMA before polymerization and the structure of nanoparticles with polymerized MMA The newly formed polymer (poly(methyl methacrylate), PMMA) is deposited on the surface of polystyrene cores of the original micelles. The thickness of the PMMA layer (8-139 Angstrom for the presented series of samples) depends on the monomer concentration. The SANS results indicate that the PMMA layer is penetrated by the poly(methacrylic acid) corona chains forming channels accessible to water.