Diffusion broadening spectroscopy has been used to make in situ size measurements of submicron aerosol particles in an acetylene-oxygen flame. Making use of the spectral broadening of scattered laser light due to random particle motion, this optical technique has been demonstrated to be suitable for application in unsteady high temperature environments of 2200 °K in flowing gases. Particle sizes were found to vary from 40 to 250 nm as the height above the burner varied from 0·5 to 3·0 cm. Little variation was found as a result of equivalence ratio changes from 2·5 to 5·0. Sizes measured from electron microscope photographs of soot particles collected from the flame show reasonable agreement with those determined from scattering measurements. A method to measure geometric mean diameter and standard deviation of the particle size distribution in a polydisperse aerosol is also presented, and experimental results described. Modifications necessary to account for polydispersity and noncontinuum flow effects are discussed.