Emulsions are of great importance to industry. They are involved in many engineering operations, including chemical reactions, extraction, emulsification and suspension polymerization, etc. However, an important problem for these processes is how to control the size distribution of the dispersed phase. Indeed, off-line analysis of the emulsion may generate uncertainties due to sampling and dilution of the product, which are likely to change the dispersion state and physicochemical properties. In this work, an on-line optical method is proposed to characterize dispersed media in real flowing conditions. This method is based on the time-analysis of back-scattered light fluctuations. The present paper deals with the development of this method and its application to dispersions of alumina in water. The results obtained with the on-line optical method are compared with those acquired by classical laser light scattering and microscopy.