The sooting characteristics of pure substituted-aromatic fuels in a wick-fed diffusion flame were studied in terms of the particle size distribution (PSD) of the soot. The temperature and PSDs were measured at the tip of flames of different heights, using a thin-wired thermocouple and differential mobility spectrometer. The temperature at the tip of the flames was found to decrease with increases in flame height. At the smallest flame height the PSDs are bimodal with a slightly larger coagulation mode, except for trimethylbenze which exhibits a large nucleation mode. For larger flame heights the coagulation mode enlarges and shifts to larger particle diameters. After the smoke point the PSDs present a single mode of particles with sizes of about 100 nm. Close to the smoke point, all fuels show a slight decrease in the rate of particle growth while the flame changes from having a closed tip to an opened soot trail. At the same flow rates, tetralin produced the largest particles, toluene and 1,2,4-trimethylbenzene produced particles with similar sizes and n-butylbenzene produced the smallest particles. This indicates that aromatics substituted with larger aliphatics tend to produce smaller soot particles. (C) 2015 Elsevier Ltd. All rights reserved.