Results are presented from tests aimed at evaluating the chemical effects of aromatic components on soot formation in laminar diffusion flames and in an experimental diesel engine, The objectives of the three sets of tests were: to evaluate the sooting tendency of (1) binary mixtures of alkane and aromatic compounds, or alcohols and aromatic compounds, consumed in laminar diffusion flames; (2) various diesel-fuel blends consumed in laminar diffusion flames; and (3) binary mixtures of alkane and aromatic compounds consumed in a diesel engine. The results from the first set of tests showed that the sooting tendency of the binary mixtures is very sensitive to the proportion of aromatic compound. However, when the proportion of aromatic compound rose beyond about 10% (by volume), the flame released soot profusely, and further increases in aromatic proportion had no significant effect. The second set of tests showed that various diesel fuels with aromatic content between 17 and 29% resulted in a very high laminar flame sooting tendency for all the diesel fuels tested. These results are in qualitative agreement with those from the first set of tests. The third set of tests, conducted in an experimental engine, showed that the amount of exhaust particulates was insensitive to the proportion of aromatic compound, after variations in ignition delay had been accounted for; this is in contrast to the results from the first set, and it suggests that the greatly enhanced air-fuel mixing present in diesel engines tends to offset the effects of fuel structure on formation of particulates.