This paper describes an apparatus for the measurement of the light transmittance of emulsions. A photoelectric technique was employed to determine the effect of flow rate, mixing time, and dispersed phase concentration on the light transmittance of a liquid-liquid emulsion composed of a commercial petroleum solvent and water. For a given concentration, light transmittance was independent of flow rate and became constant after 10 minutes of mixing in the test apparatus. Light transmittance decreased with solvent (dispersed phase) concentration. Up to 5% solvent concentration, the light transmittance-concentration curve could be used to measure solvent concentration accurately.