Abstract: A system and method are disclosed for the self-calibrating, on-line determination of size distribution and volume fraction of a number of particles dispersed in a medium by detecting multiply scattered light from the particles. The multiply scattered light is re-emitted in response to exposure to a light source configured to provide light of time varying intensity at selected wavelengths. The determination includes calculating the isotropic scattering coefficient for the particles at each of a number of wavelengths from the multiply scattered light as a function of an intensity modulation phase shift, and iteratively estimating the size distribution and volume fraction as a function of the isotropic scattering coefficient for each of the wavelengths. An estimation approach based on an expected form of the distribution and the mass of the particles is also disclosed.