Emulsion polymerization is one of the major techniques for the manufacture of adhesives, coatings, thermoplastics and elastomers. In miniemulsion polymerization, relatively stable oil droplets within a size range of 500 to 5000 angstrom are prepared by emulsifying a monomer in a medium, generally water, with the aid of a surfactant and a hydrophobic compound. Droplet size and size distribution are by far the most important parameters of miniemulsion because they affect directly both the miniemulsion stability and droplet nucleation. Therefore, the understanding of the mechanism ruling miniemulsion polymerization strongly depends on an accurate determination of the particle size. Small-angle neutron scattering has been used for the polymerization study of the 1,3,5-tris(trifluoropropylmethyl)cyclotrisiloxane (F-3). In particular, the shape and the size of the particles before and after the anionic polymerization were sought. Surprisingly, the sizes obtained are of the order of 250 angstrom which ranges this system rather into the microemulsion domain and the observed growth in the mean particle size (25%) implies that there is no full preservation of the particles during the polymerization reaction. On the other hand, the contribution to the scattering of a second population of smaller particles (38 angstrom) with a broad size distribution in the polymerized sample is attributed to secondary products leading to a 80% polymerization reaction yield.