The effects of fuel dilution by CO2 and N-2 on soot formation and the flame structure in laminar coflow C2H4/air diffusion flames at pressures between 5 and 20 atm were investigated both experimentally and numerically. Experimentally a constant ethylene flow rate and a constant dilution rate of 1:2 (fuel:diluent by mass) were maintained throughout the experiments. The flames were stable and non-smoking over the pressure range investigated. The radially-resolved soot volume fraction and temperature distributions were measured by the spectral soot emission (SSE) technique. Numerical calculations were conducted using two C-2 chemistry models with formation of PAHs up to pyrene and a soot model incorporating pyrene collision as the soot inception step and hydrogen-abstraction acetylene addition mechanism and PAH condensation as the surface growth processes. The two C-2 chemistry models were the ABF mechanism (Appel et al. (2000)] and the DLR mechanism (Slavinskaya and Frank (2009)]. The DLR mechanism predicted little or no chemical effect of CO2 dilution, depending on the pressure, in the present context. Numerical results are in qualitative agreement with experimental measurements. Soot volume fractions and carbon conversion are lower in the CO2-diluted flames due to the additional chemical effect of CO2. CO2 is still more effective than N-2 as a diluent to suppress soot formation at elevated pressures. The primary pathway for the chemical effect of CO2 dilution is through the reverse reaction of CO + OH <-> CO2 + H. The chemical effect of CO2 lowers the rates of soot inception, C2H2 addition, and PAH condensation. The effectiveness of the CO2 chemical effect on soot formation suppression diminishes with increasing pressure. The diminishing effectiveness of the chemical effect of CO2 dilution with increasing pressure is due to the significant decrease in the H radical mole fraction. Crown Copyright (C) 2015 Published by Elsevier Inc. on behalf of The Combustion Institute. All rights reserved.