The soot formation characteristics in laminar premixed flames of pure n-heptane and binary mixtures of toluene and n-heptane with liquid volume ratios ranging from 0.2 to 1 were studied with the C/O ratio and unburned gas-mixture velocity being kept the same for all tested flames. The particle size distribution functions (PSDFs) at several selected burner-to-stagnation surface separation distances (Hp) were measured by using the burner-stabilized stagnation probe/scanning mobility particle sizer (SMPS) technique. In addition, the morphology of soot particles sampled from the probe was examined using transmission electron microscopy (TEM). From the PSDFs at different Hp and TEM images, it was observed that with the addition of toluene, soot inception occurred at lower flame heights and the primary particle size of soot aggregates was significantly reduced. A combustion kinetics model for toluene and n-heptane was used to explore the precursor chemistry. The modeling results were found to be consistent with the observations of the measured PSDFs.