The effects of adding H-2 to fuel on the formation of soot are studied numerically and experimentally in a laminar coflow C2H4 diffusion flame burning in 30%O-2 -70%CO2 atmosphere by applying the open source flame code CoFlame and two-dimensional light-of-sight attenuation (2D-LOSA) diagnostic, respectively. H-2 and He addition to C2H4 were employed in experiments to discuss the chemical effect of adding H-2. An extended ABF mechanism with inclusion of two virtual species fictitious hydrogen (FH2, inert H-2) and fictitious carbon dioxide (FCO2, inert CO2) was used to model the gas-phase chemistry and to isolate the chemical effects of H-2 and CO2 in numerical calculations. Both numerical and experimental results show that the flame height remains nearly unchanged with addition of H-2 and He up to 50%. The chemical effect of both CO2 and H-2 suppresses soot loading in all the C2H4 flames investigated. Hydrogen addition to C2H4 increases mole fractions of H and H-2, but lowers the mole fractions of C3H3 and polycyclic aromatic hydrocarbons (PAHs) and suppresses soot nucleation directly soot surface growth process indirectly. Carbon dioxide in the oxidizer suppresses soot formation by mainly weakening the soot surface growth process through reducing the H radical concentration and flame temperature and secondarily enhancing the oxidation process due to higher mole fraction of OH. There is a weak chemical coupling effect between H-2 in fuel and CO2 in oxidizer to further suppress soot formation. This chemical coupling effect results in a lower mole fraction of H, which in turn reduces both soot nucleation and subsequent growth process through surface reactions. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.