An experimental assessment of the influence of carbon dioxide and nitrogen dilution on sooting characteristics of laminar ethylene diffusion flames at pressures up to 20 atm is presented. Two dilution rates, defined as the ratio of mass flow of the fuel to that of the diluent gas, of 1:2 and 1:3 were used at all pressures with a fixed ethylene mass flow rate. A wider range of nitrogen dilution, from 1:1 to 1:4, was investigated at 10 atm. In the pressure range of interest and with the mass flow rates of fuel and diluents, resulting flames were stable and nonsmoking. Spectrally-resolved line-of-sight soot radiation measurements were obtained to infer the radial soot and temperature distributions within the flame envelope through an Abel inversion process. The sooting propensity, in terms of maximum soot yield, was found to be significantly lower with carbon-dioxide dilution in the pressure range of 1-15 atm but approached to comparable values to those with nitrogen-dilution at 20 atm. The implication of this finding is that the chemical suppression effect of carbon dioxide dilution, which was proven at atmospheric pressure previously, exists also at elevated pressures up to 15 atm and becomes relatively small at higher pressures. Variation of the maximum soot yields with pressure indicated that carbon dioxide-diluted flames show a relatively stronger dependence to pressure as compared to nitrogen-diluted flames. Temperatures decreased with increasing pressure as expected due to increasing, radiative heat loss, and the peak temperatures were observed near the flame tips as a result of the heat release from soot oxidation. (C) 2017 Elsevier Ltd. All rights reserved.