A novel type of three-layer nanoparticles was studied using small-angle neutron scattering (SANS). The particles were prepared by adding methyl methacrylate monomer to a polystyrene-block-poly(methacrylic acid) micellar solution in aqueous buffer and subsequent polymerization by gamma -irradiation. The contrast-matching SANS experiments revealed that upon polymerization, the PMMA chains form a layer on the surface of the PS cores of the original micelles. This is in agreement with the finding that at room temperature, MMA monomer molecules do not penetrate into the micelle core in detectable amounts. In the presented example, the mean core radius of original micelles and the layer thickness were determined to be 99 and 17 Angstrom, respectively. Both characteristics are expected to be controllable by the choice of block copolymer micelles and the parameters of the polymerization process. (C) 2001 Elsevier Science Ltd. All rights reserved.