An experimental study has been conducted to investigate soot formation in laminar co-flow ethylene-air diffusion flames with nitrogen dilution from a co-flow circular nozzle at pressures from 10 to 35 atm. Spectral soot emission (SSE) diagnostic technique was used to determine the radially resolved soot and temperature field structure. Constancy of ethylene and nitrogen flow rates were maintained and the flow rates of ethylene and nitrogen were selected such that no smoke was emitted even at the highest soot loadings. The flame height, marked by visible flame radiation, remained constant at about 5.5 mm and the cross-sectional area of the flame decreased with increasing pressure. At 10 atm, the peak soot concentration of less than 8 ppm, was measured near the flame tip. At 35 atm, the peak soot concentration of about 62 ppm, was measured near the mid-height of the flame. The conversion of carbon in the fuel to soot was strongly dependent on pressure particularly in the lower pressure range. At higher pressure this dependence was weaker. The peak carbon conversion to soot, 6.5%, was observed at 30 atm and remained constant to 35 atm. Temperatures increased along the flame axis and the peak temperature was observed near the flame tip to indicate complete soot oxidation. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.