The sooting tendency of gaseous and liquid hydrocarbon fuels has been determined systematically in an axisymmetric laminar diffusion flame whose temperature was controlled by nitrogen dilution. Sooting tendency was measured by the minimum mass flow rate of fuel (FFM) at the smoke height. Results, plotted as log 1/FFM versus l/T, where T is a calculated adiabatic flame temperature, show that fuel structure plays a significant role in diffusion flames. Comparison of these flame results with basic pyrolysis studies in the literature supports the concept that pyrolysis of the fuel molecule is a controlling factor in determining the overall tendency to soot, even though such tendency results from the competition of pyrolysis of the fuel and heterogeneous oxidation of the. soot particles. The pyrolysis characteristics affecting the sooting process are rate, sequence and nature of products, and pyrolysis mode (pure or oxidative). The aromatics show a temperature sensitivity with respect to sooting tendency significantly lower than the other fuels. Conjugation of the initial fuel molecule and pyrolysis intermediates enhances sooting propensity.