In situ optical diagnostics have been more widely used in recent years for measurements in both research and industrial facilities. The subject of this paper is an in situ particle counting and sizing technique that uses the intensity deconvolution method. This technique is also capable of simultaneously measuring the velocity of a particle and therefore allows absolute particle concentrations to be obtained. The technique can be used in both the forward scatter and backscatter geometries. This paper first describes the technique and presents the theory of particle sizing by the intensity deconvolution method. An example of the successful measurement of particle size distributions in an industrial reactor using the backscatter configuration is given, and the results are discussed. Measurements using the forward scatter configuration of particle size distributions and velocities for coal/water slurry combustion are described. A discussion of the size resolution, accuracy, and error analysis of the technique is presented.