We investigate the sooting propensity of an Alcohol-to-Jet-Synthetic Kerosene with Aromatics 12 (ATJ-SKA) biojet fuel. The soot volume fraction and primary particle size in the pre-vaporised 13 diffusion flames using ATJ-SKA biojet and blends with Jet A-1 at atmospheric conditions were 14 measured experimentally and compared to numerical simulations. The measurements were 15 conducted using extinction calibrated laser induced incandescence (LII). Within the 10% 16 measurement uncertainty, the soot volume fractions measured using ATJ-SKA fuel do not show 17 significant difference with measurements with Jet A-1. A comparison of the chemical 18 composition of the fuels suggests that the Degree of Unsaturation (DoU) may not determine the 19 sooting propensity of biojet fuels. The SEM analysis shows that diffusion flames using neat Jet 20 A-1 produce finer soot particles and larger number density of compared to biojet and biojet 21 surrogate. The soot model employs a semi-detailed chemical kinetic mechanism and a physical 22 model which integrates the population balance equation governing the soot particle size 23 distribution with the in-house reactive flow solver for multicomponent ideal gases. The model 24 predicted the soot maximum soot volume fraction (SVFm) in the neat biojet case and the blended 25 cases with Jet A-1 fuels within an error margin of 13% comparing with the measured values. 26 However, the predicted a soot volume fraction distribution patterns differ with the measured 27 ones and the possible reasons are discussed.