The effect of aliphatic side chains in the feedstock on soot formation was studied with experimental and numerical simulation. Feedstocks of ethylbenzene, xylene, toluene, and benzene were pyrolyzed at different residence times and temperatures, and the particle size distribution (PSD) of the formed soot was measured. The initial nucleation rates of soot during ethylbenzene or toluene pyrolysis was high. However, after a long residence time, the amount of soot produced from benzene pyrolysis became higher than for these feedstocks. This result suggests that at long residence times, the growth rate of soot from benzene is higher than those of ethylbenzene and toluene. Numerical simulation for the pyrolysis of ethylbenzene, toluene, and benzene was conducted with the KAUST PAH Mechanism 2 (KM2) model, and the prediction accuracy was improved by adding some new reactions. The simulations were performed at two conditions similar to the experimental ones: The production rate of polycyclic aromatic hydrocarbons (PAHs) was very high at the beginning of reactions of ethylbenzene and toluene, indicating high rates of soot nuclei generation. In benzene pyrolysis, modification of the KM2 model led to a higher predicted PAH concentration than that with the original KM2. Most of the reactions we added to the KM2 model are those of PAH growth by phenyl addition (PA). Therefore, the PA mechanism would be an important reaction pathway for PAH growth in the pyrolysis of benzene. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.