A light scattering method has been devised to discriminate between coal and fly ash and measure particle size. Sizing was performed by the measurement of forward scattered intensity, and discrimination was displayed by the ratio of the intensities of crossed to parallel polarisations at a scattering angle of 160-degrees. The technique has been applied to a study of coal combustion in a variable length drop tube furnace. It has been demonstrated that the development of the size distribution can be followed as a function of residence time. Further, it is indicated that there is a simple correlation between the polarisation ratio and the carbon content of the ash. This leads to the possibility of monitoring combustion efficiency in industrial furnaces.