Application of Rayleigh spectroscopy for characterization of particle size in nonaqueous dispersion and water-based emulsion paint resins is described. The technique allows a straightforward and rapid estimation of particle size; the measurement does not require exact determination of scatterer concentration. For monodisperse samples, unambiguous results are obtained for particles at least up to 50 μm in diameter; for polydisperse samples, an average size heavily weighted by large particles is obtained. Typical experimental results on monodisperse and polydisperse water-based latexes and on polydisperse nonaqueous dispersion resins are described. In the latter case, comparison of electron micrograph and light scattering size determinations indicates that the light scattering experiment yields approximately a z-average radius. Observations on particle formation and growth during polymerization are also described.