In this study, growth mechanisms of polycyclic aromatic hydrocarbons (PAHs) leading to soot formation were discussed. In addition, the effect of PAHs on soot nucleation was investigated by comparing the concentration of PAHs under sooting and non-sooting conditions. The feedstock ethylene was pyrolyzed in an isothermal laminar flow at 1150-1730 K with a residence time of 16-363 ms. Reaction pathways of PAH growth at each temperature were discussed using total ion chromatograms (TICs) of gas chromatography/mass spectrometry (GC/MS). These TICs suggest that the hydrogen-abstraction carbon-addition (HACA) and methyl addition/cyclization (MAC) mechanisms are the main mechanisms for large PAHs at 1350 K and that phenyl addition/cyclization (PAC) also plays an important role at 1730 K. This paper was the identification of pathways previously rarely considered in soot models: the pathway via dimerization of acenaphthylene; the pathway from perylene to coronene; the pathway from tetraphene to benzo[k]tetraphene. Some significant peaks of aliphatic hydrocarbons were also detected in the TIC at 1730 K. The quantification of pyrolysates was examined by GC/MS with deuterides. At 1730 K, the number concentration of nascent soot increased along with the mole fraction of PAHs. Despite a relatively high mole fraction of PAHs with molecular masses of 200-300 u, no soot was observed at 1150 K. This indicates that such PAHs are not precursors of nascent soot. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.