Soot volume fractions have been measured using optical techniques in dual mode diffusion flames. A dual mode flame is defined as a diffusion fuel jet that burns in the wake of a premixed flame. The premixed flame combustion products simulate a vitiated-air environment: the air is preheated, rich in radicals, and depleted in oxygen. A two stage burner was used to investigate the effects of vitiation on the soot yield in diffusion flames. In the first stage, a fuel-lean premixed methane flame burned. A diffusion fuel jet was injected in the second stage. The effects of vitiation were simulated by varying the premixed flame stoichiometry, diffusion flame injection height above the premixed flame, and premixed flame temperature. The premixed flame stoichiometry affects the oxygen availability. The oxygen availability was varied from an 18% oxygen index to a 23% oxygen index. Different diluent gases (Ar, N₂, and CO₂) were used to alter the premixed flame temperature. The injection height of the diffusion flame (0.0 to 1.0 cm) affects the radical concentrations and temperature field. The soot yield from dual mode combustion was compared to the yield from similar single mode flames. For the single mode flames, air was injected through the first stage. Experimental results indicate that dual mode combustion increases the soot yield significantly in diffusion flames. Among flames at a 23% oxygen index, the mean increase in soot levels was 84%. Among flames at an 18% oxygen index, mean soot levels increased by 314%. In dual mode flames, the effect of injection height is weak, as is the effect of oxygen availability. The effects of the feed air temperature could not be resolved. In single mode flames, the effect of the injection height was weak, but oxygen availability affects soot yield significantly. Single mode flames show an average increase of 180% in soot yield at the 23% oxygen index with respect to the 18% oxygen index. These conclusions are the same for both ethene and propane as the diffusion flame fuels. / Master of Science