A sooting, ethylene coflow diffusion flame has been numerically studied. A detailed kinetic mechanism, which includes 60 chemical species and 279 reactions, has been coupled with transport equations and solved numerically. An important aspect of the mechanism is the role of resonantly stabilized radicals, that is, propargyl, cyclopentadienyl, and benzyl radicals, in addition to hydrogen abstraction acetylene addition, to model the growth of aromatics. The model predicts temperature profiles and flame height reasonably well compared to the experimental measurements. Also, the predicted peak mole fraction of acetylene is in good agreement with experimental data both in the maximum value and its location. The formation of benzene is predicted with agood level of accuracy. The modeling shows that in diffusion-controlled conditions, such as in premixed flames, benzene formation is controlled by propargyl radical combination. Key reactions leading to the formation of larger aromatics are the combination of resonantly stabilized radicals. The model also correctly reproduces the formation of particulates sampled by thermophorectic techniques, that is, soot and its high-molecular-mass precursors. Although uncertainties in the rate constants of some reactions will affect the prediction of intermediate species, it is worth noting that the model is able to correctly reproduce the concentration profiles of major and trace species.