Abstract The emission standards of combustion have been steadily reduced in recent years, and a large research effort has been focused on lowering the emissions of hydrocarbons and particulate matter. Addition of oxygenates to fuel reduces these pollutants. In this paper, we report on a detailed investigation of the growth mechanisms for gas-phase species in ethylene/air and ethylene/ethanol/air flames in order to assess the importance of various chemical mechanisms for the molecular growth of soot precursors. We employ a variety of computational techniques that include stochastic and deterministic methods to study the formation of gas-phase species and their growth into soot precursors via chemical and physical mechanisms. We have drawn the following conclusions: 1) the chemistry of oxygenated compounds (specifically the high concentrations of O 2 and OH) is critical to reproduce the experiments; 2) using free energy simulations, we found out that the tendency of molecules to form dimers is mainly affected by the molecular shape rather than the mass of the aggregate. Finally we propose a different mechanism for the growth of soot precursors based on radical–radical recombination to form molecules of high molecular masses (>1200 u). These structures are then likely to promote physical aggregation to further the growth mechanism.