Effects of CO2 on soot formation during ethylene pyrolysis were investigated in a laminar flow reactor with the addition of various amounts of CO2 (0-99.5% in mole fraction). Based on a quantitative dilution sampling technique and a scanning mobility particle sizer, soot particle size distributions and the associated global properties, including soot volume fraction, number density, and soot induction delay time, were examined. Results show that while addition of a small amount of the CO2 (0 - 10%) tends to promote soot formation as the total number and volume of soot particles increase and the soot induction delay time decreases, its further increase, from 10% to 99.5%, leads to an obvious reduction of the soot nucleation and mass growth rates. Subsequent kinetic modeling of the gas-phase chemistry showed that with increasing CO2 concentration, the corresponding concentrations of the soot precursors, namely benzene and pyrene, first increase and then decrease, which is consistent with the observed trend in soot formation. Further sensitivity and reaction path analyses of benzene formation indicate that CO2 addition produces more hydroxyl radicals, such that while the presence of a small amount of hydroxyl radicals increases the propargyl concentration and thereby promotes the formation of soot precursors, excessive hydroxyl radicals lead to more oxidation and hence inhibit soot formation. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.