A database of fuels containing carefully graded amounts of aromatic and oxygenated compounds were used to investigate the effect of the molecular structure of the fuel on the sooting tendency of laminar diffusion flames. Clear trends were found showing an increase in sooting tendency with increased aromatic content, with emphasis on the number of benzene rings. Oxygenated compounds such as diglyme were found to reduce the sooting tendency. These trends tallied closely with trends in soot emissions from a CFR IDI (Co-operative Fuel Research Indirect Injection) diesel engine, pointing towards an observable effect of the fuel on emissions of soot from engines. Laser techniques were used to measure the sooting tendency of the flame, by aiding measurement of the threshold sooting height and by laser extinction and laser-induced incandescence (LII) imaging. The UI technique allowed more detailed investigation of the sooting tendency through quantitative visualisation of the soot distribution within each frame.