Abstract: The mean particle size of a dispersed phase, such as a liquid dispersed in a carrier liquid, is determined by selecting a particulate solids material which, when dispersed in the carrier liquid, has the same refractive index as the dispersed liquid in the carrier liquid, measuring the turbidity of selected particle sizes of the particulate solid for different concentrations of each particle size in the carrier liquid and developing a relationship of the ratio of the turbidity to the concentration for each particle size as a function of particle size which may be referenced when measuring the turbidity and concentration of the dispersed liquid in the carrier liquid to determine the actual particle size of the dispersed liquid.