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Previous soot studies in counterflow diffusion flames revealed that the sooting limit curve in regions with large oxygen mole fractions (X_O) exhibited marked bending behaviors that indicated a non-monotonic variation of sooting tendency with oxygen concentration. The underlying mechanisms of this bending behavior remained unclear. In this regard, the present study systematically investigated the effect of oxygen mole fraction in the oxidizer stream on the sooting characteristics of ethylene counterflow diffusion flames. We used the near-infrared light extinction technique to measure soot volume fractions of two types of flames that significantly differed in sooting structures: soot formation oxidation (SFO) flames with a fixed fuel mole fraction (X_F) of 0.28 and varying X_O between 0.5 and 1.0 and soot formation (SF) flames with pure ethylene (X_F = 1.0) in the fuel stream and varied X_O from 0.25 to 0.3. We also …