The effect of pressure on sooting behaviour of n-heptane is studied in co-flow n-heptane/air laminar diffusion flames at pressures above atmospheric in a high pressure combustion chamber. The fuel is diluted with either nitrogen or helium to keep a non-smoking flame at elevated pressures, and the selected fuel mass flow rate of n-heptane provided diffusion flames in which the soot was completely oxidized within the visible flame envelope. The flame stability proved to be a challenge and stable flames were possible only at certain pressures for a sufficiently long duration to permit measurements. The soot volume fractions and temperatures were measured by spectral soot emission as a function of pressure for nitrogen-diluted n-heptane flames at 2, 5 and 7 atm. For helium-diluted n-heptane flames, line of sight soot emission data at 3, 4, and 5 atm are presented at two heights above the burner exit. Comparison of limited nitrogen-diluted n-heptane data to previous measurements of soot yields indicate that soot formation in diffusion flames of n-heptane seems to be slightly more sensitive to pressure than that in aliphatic gaseous fuel diffusion flames within the pressure range considered in this work. (c) 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.