An experimental investigation has been conducted on the influence of pressure on soot formation in counterflow diffusion flames of nitrogen-diluted ethylene versus air from 1 to 2.5 atm. By measuring the sooting strain rate limits in counterflow flames while maintaining the same flame temperature through argon substitution, the influence of pressure on soot inception/growth is isolated. Results demonstrate that the sooting tendency is enhanced at elevated pressures, with its magnitude being of the same order as that of the fuel dilution effect. The density-weighted strain rate at the sooting limit is found to vary linearly with pressure, and hence suggesting that the overall soot inception/growth reaction is of first order within the pressure range studied. Through numerical simulation of the detailed flame structure, the characteristic dependence of the sooting limit on pressure and fuel concentration is found to be consistent with mass growth processes dominated by the acetylene reactions.