The soluble organic fraction (SOF) of particulates emitted from a high-speed four-cycle DI diesel engine at various loads for commercial diesel fuels and some blended and distillate fuels is studied under steady operating conditions. The fuels examined have various final boiling points and aromatic content. Emissions of lubricant, fuel fractions, and combustion products in the SOF are evaluated by chemical analysis. Exhaust emission data indicates that the addition of aromatic hydrocarbons to fuel does not cause increases in SOF or in solid fraction, and that heavier components of fuel are responsible for high level of SOF emission at medium load mainly due to the deposition of fuel on the wall of the piston cavity. Furthermore, flow reactor experiments under atmospheric pressure have been conducted to look at the temperature effect of forming SOF. The results successfully explain the dependence of SOF emission on load and fuel properties.