The systematic investigation of the effect of two major experimental parameters (equivalence ratio and temperature) as well as fuel structure on the sooting limits of ethylene, propane, and ethylene/toluene flames was carried out in premixed burner-stabilized stagnation flames through qualitatively visual observations of flame yellow luminosity and quantitatively direct measurements of incipient soot particle size distributions by a scan mobility particle sizer. A strong temperature dependence of incipient soot formation was observed in the three flames. Detailed chemical kinetic modeling of the flame structure and the soot precursor chemistry was performed to explain the experimental observations. The results show that the incipient soot formation is prohibited in either low or high temperature flames. While the former is due to slow reactions induced by both low temperatures and small concentrations of soot precursors, the latter is attributed to the thermodynamic reversibility of polycyclic aromatic hydrocarbons (PAHs) at high temperatures. Sooting limits have been found to be not a function of concentrations of PAHs alone at a fixed flame temperature; sooting propensity generally increases with increasing fuel carbon/hydrogen atom ratio, presenting the following order: propane < ethylene < ethylene/toluene.