A kinetic mechanism, previously developed and successfully applied to predict the formation of benzene and larger aromatics in pre-mixed flames is applied to co-flowing diffusion flames. The mechanism emphasizes the role of resonantly stabilized radicals, in addition to the acetylene addition mechanism (HACA mechanism). The flames modeled are atmospheric pressure, axisymmetric, co-flowing laminar diffusion flames with varying amounts of oxygen added to the fuel, as studied by McEnally and Pfefferle. The model predicts, with a good level of accuracy, the growth of hydrocarbons and the formation of benzene and aromatic species. The results show that in diffusion-controlled conditions. as in pre-mixed 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, including propargyl addition to benzyl radicals and to a lesser extent cyclopentadienyl radical combination. The mechanism of acetylene addition to aromatic rings (HACA mechanism) contributes negligibly to the formation of larger aromatics. The model also explains the effect on the formation of non-fuel hydrocarbons of pre-mixing oxygen with the fuel. In particular, the model shows that partial pre-mixing shifts the pyrolysis mechanism of the fuel toward odd-carbon hydrocarbons. This in turns leads to enhanced production of benzene and larger aromatics, because of the importance of reaction mechanisms involving resonantly stabilized radicals with an odd number of carbon atoms, such as propargyl (C3H3), cyclopentadienyl (cC(5)H(5)) and benzyl (C7H7) radicals, in forming aromatics.