The advantages of asymmetrical flow field-flow fractionation (AF-FFF) for the characterization of aqueous dispersions of spherical polyorganosiloxane nanoparticles are discussed. With AF-FFF it was possible to obtain information about the synthesis, which is based on the hydrolysis and condensation of alkylalkoxysilanes in aqueous dispersion, and the average size of the spherical nanoparticles in the complex mixture in the presence of excess surfactant. The results are compared to measurements performed with dynamic light scattering (DLS). The size of the nanoparticles increases as a function of the amount of added monomer. Particles with radii between 2 and 50 nm are observed. If only the crosslinking monomer methyltrimethoxysilane (T) or a fixed monomer mixture of T and the chain-forming monomer diethoxydimethylsilane (D) is used, the increase in the radius shows a cube dependence on the volume of added monomer as expected. With AF-FFF it is also possible to obtain information on the role of the surfactant, which is needed to stabilize the particles. For spherical nanoparticles that are composed only of the trifunctional T or of a monomer mixture of T and D, it was found that the amount of surfactant needed to stabilize the growing particles is proportional to their surface. In the case of a more complex spherical core-shell architecture it was possible by AF-FFF analysis to obtain information about a secondary nucleation which may take place during the synthesis.