The transport properties of particulate process streams and their final product quality, are directly affected by critical parameters of particle size distribution, f(x), and volume, mass, or number density of particles or dispersed phase droplets. A method is proposed for the potential on-line monitoring of particle size distribution and volume fraction in real time, using frequency domain photon migration measurements (FDPM). Theory, experimental measurements, and results for the determination of particle size distributions fur both a polystyrene latex and a titanium dioxide suspension determined using the photon migration technique are presented. The critical issues associated with the application of photon migration to particulate and dispersed phase processes are discussed, including the effects of interparticle interactions on the transport of light.