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This study presents the sooting tendencies exhibited by binary mixtures of synthetic fuels i.e. n-heptane/toluene and isooctane/toluene, in terms of Yield Sooting Indices (YSIs). These synthetic fuels are indeed incorporated into almost every gasoline surrogate fuel used for experimental study or performing numerical simulation of combustion process in engines. Diffusion flames of methane diluted by nitrogen and doped with the vapors of the aforementioned mixtures are established over the Santoro’s burner. Two-dimensional maps of soot volume fraction in these flames are then inferred from line-of-sight attenuation measurements. The measured soot volume fractions are converted into apparatus-independent YSIs which affords the possibility to compare the relative influence of fuel formulation on soot production. The trends in soot production capacity as a function of toluene mole fraction are then analyzed by curve fitting YSI data. A non-linear evolution of the sooting tendency as a function of toluene mole fraction in the binary mixtures reveals especially for isooctane/toluene blends at lower toluene mole fraction. Two Quadratic equations adequately model the evolution of YSI as a function of toluene mole fraction in n-heptane/toluene and isooctane/toluene blends. The trends as inferred from these equations and experimental results are in fair agreement with earlier studies, which further sustain the consistency of the YSI methodology. Interestingly, in an earlier study[Combust. Flame 161 (2012) 1575-1586], almost linear evolution of sooting tendencies has been evidenced for the blends of n-heptane and isooctane. This clearly points towards the presence of a synergistic effect only visible when certain types of hydrocarbons are present in the fuel mixture (branched-chain alkanes and alkyl benzenes in our case). The quadratic equations presented here can be very helpful while formulating fuel mixing strategies in view of the future engine and environmental exigencies and can provide preliminary predictions of soot emissions characteristics of a fuel.