Abstract: A method of measuring sizes of particles suspended in a liquid using optical mixing spectroscopy of scattered light and an apparatus for implementing the method are disclosed. The method involves making measurements by a fiber optic probe introduced into a medium under investigation, the probe including several multiple or single mode optical fibers. One of the optical fibers transmits light into the medium, while the other optical fibers transmits scattered light to a device for providing spatial coherence of the light and then to a light detector. Auxiliary optical fibers are used to determine the relationship between spectral line widths of multiple and single light scattering when measurements are conducted in very turbid media and for determining particle sizes from multiple rather than single scattering spectrum.