Varying oxygen concentrations was found to have pronounced effects on the sooting characteristics of diffusion flames. Ethylene and propane flames were stabilized around a porous ceramic cylindrical burner mounted in a vertically oriented, low turbulence wind tunnel. Laser light scattering and extinction, laser Doppler velocimetry and thermocouple temperature measure- ments were obtained along the forward stagnation line. Measurements of soot particle size, number density and volume fraction showed that with an increasing oxygen index the soot volume fractions in both ethylene and propane flames increased at all locations along the stagnation stream- line. For ethylene flames (when the oxygen index increased from 0.18 to 0.28). the maximum soot volume fraction increased by almost an order of magnitude. Propane was found to produce about one half the volume of soot of an ethylene flame at the same oxygen index. The increase in soot volume fraction with increasing oxygen index was attributed to both an increase in the formation of particles and in the subsequent rate of surface growth. The data have yielded information on rates of surface growth. In addition, an apparent activation energy of 24 kcal/mole has been determined for the surface growth reactions in the ethylene flames.