Regulations related to soot emissions are becoming more stringent due to the negative impact of soot on the environment and human health. Thus, not only the total mass of soot released from hydrocarbon combustion have to be controlled but also its particle size distribution. Combustion technologies such as oxygen-enriched and oxy-fuel allied with flue gas recirculation have demonstrated their potential for reducing pollutants while improving combustion efficiency. In this context, this work explores the effect of CO2 addition on the soot formation process under an oxygen enriched atmosphere. A set of laminar counterflow ethylene flames are numerically studied for CO2 addition on either the fuel or the oxidizer mixtures for the same CO2 amount in the reaction layer. The numerical approach accounts for detailed chemistry and transport properties, together with an advanced model for thermal radiation and the discrete sectional model for soot formation. It was observed that CO2 suppresses the formation of species such as hydrocarbon radicals (C2H, C3H3), C2H2 and A1, which are important PAHs building block species directly involved in the soot formation process. For the same amount of CO2 in the reaction layer, it was found a more expressive suppression of the aforementioned species concentrations when CO2 was added to the fuel mixture. Moreover, for the same amount of CO2 in the reaction layer, it was also found that while chemical effects played a major role on the formation of C2H2, C3H3 and PANs for the CO2 addition on the oxidizer side, both chemical and thermophysical effects are important for CO2 addition on the fuel side. In both cases, the temperature profile was mainly influenced by thermophysical effects while the soot volume fraction was mainly influenced by chemical effects. Finally, the particle-size distribution reveled to be strongly bimodal and slightly sensitive on the CO2 addition for the current flames. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.