—The isolated effects of fuel dilution and flame temperalure variation on soot formation have been investigated experimentally for counterflow dilTusion flames. The methodology of isolation through temperature adjustment involves changing the concentration of the fuel by diluting it with nitrogen, and then incrcasing the maximum temperature of the diluted flame back to that of the undiluted Rame by replacing a portion of the nitrogen in the oxidizer flow with an equal portion of argon. Soot quantities are determilied by using light-scattering and extinction techniques, flame temperatures by thermocouple measurements, and velocities by LDV mapping. Results on ethylene show that both temperature and dilution contribute to a reduction in the soot volume fraction and dilution accounts for a substantial amount of this reduction. I t is further found that the soot formation rate and specific surface area are linearly dependent on the initial fuel concentration, while the specific surtace growth rate is insensitive to its variation. Finally, the soot formation process is insensitive to a wide range of fuel ejection rates for the present counterflow diffusion flame established in the forward stagnation region of a porous cylinder.