Frequency-domain measurements of photon migration were conducted in concentrated polystyrene suspensions to assess their use for on-line particle sizing in undiluted process streams. Using a numerical inverse algorithm, particle-size distributions (PSD) and volume fractions of three latex suspensions with differing distributions were recovered from phase-shift measurements at 15 or less visible wavelengths. Comparison to dynamic light-scattering measurements show excellent agreement. Since it is the propagation characteristics of multiply scattered light instead of the amount of light detected that is employed to solve the inverse problem, the measurement technique provides a self-calibrating (and therefore especially suitable) method for on-line process monitoring of PSD in the chemical-based industries.