A seeded surfactant-free methyl methacrylate emulsion polymerization is employed to prepare coreshell particles with predetermined size and narrow size distribution. The particle size is determined by the ratio between methyl methacrylate (MMA, shell) and polytetrafluoroethylene (PTFE, core). Monodisperse particles in the 100350 nm range are obtained. These particles are used to grow colloidal photonic crystals (opals) of very high optical quality, thus indirectly demonstrating the excellent control of microbead size distribution achieved by this preparation technique. The optical properties of the opals are investigated by means of reflectance and polarized-angle-resolved transmittance spectroscopies. These data provide a rough determination of the effective refractive index of the system, which is favorably compared with values obtained by simple effective medium models. Copyright (c) 2012 Society of Chemical Industry