Experimental results are presented for changes in soot volume fraction resulting from the addition of a JP-8 surrogate and each of its components to a N-2-diluted, C2H4 co-flow diffusion flame. The surrogate, which consists of 77% n-dodecane and 23% m-xylene by volume, was designed to match the threshold soot index of a nominal JP-8 fuel. Total carbon flow rate was constant for all experiments: pre-vaporized liquid fuel was added at two different levels: 2.5% and 5% of the total carbon flow rate. Tests were conducted at pressures from 1 to 5 atm. The use of relatively small amounts of carbon from the liquid fuel resulted in a linear relationship between the peak soot volume fraction and the amount of carbon from the liquid fuel. In these experiments, the peak soot volume fraction was found to be vary with pressure according to a power-law relationship, consistent with prior work on pressure effects on soot. The surrogate fuel showed very similar trends to the JP-8, but yielded lower soot volume fractions. Simulation results for the flames with m-xylene capture the trends of increasing soot volume fraction with increasing carbon from the liquid fuel and with increasing pressure. However, the simulations show smaller increases than were observed in the experiments. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.