The effects of adding hydrogen and nitrogen on soot concentration in atmospheric laminar ethylene jet flames are studied computationally and computed results are compared with experimental ones. Two sets of flames are studied: one in which the total volumetric injected flow rate is kept fixed and ethylene is substituted with either hydrogen or nitrogen and the other in which the ethylene volumetric flow rate is fixed but varying amounts of hydrogen and nitrogen are added. A two-equation semi-empirical model is employed to model soot. The model is able to qualitatively predict the measured trends of reducing soot concentration as hydrogen or nitrogen is added to ethylene or substituted. Quantitative agreement is within an order of magnitude and the results are found to depend strongly on the coupling between radiation and soot. (C) 2018 Elsevier Ltd. All rights reserved.