A prototype 1991-model diesel engine was tested using EPA transient emissions procedures to determine the effect of fuel properties on combustion characteristics and exhaust emissions. The eleven test fuel set focused primarily on total aromatic content, multi-ring aromatic content, and cetane number, but other fuel variables were also studied. Hydrotreating was used to obtain reductions in fuel sulfur and aromatic content.Increasing cetane number and reducing aromatic content resulted in lower emissions of hydrocarbons and NOx. Particulate emissions were best predicted by sulfur content, aromatic content and 90% distillation temperature. Multi-ring aromatics showed a greater significance than total aromatics on hydrocarbon and particulate emissions. Combustion parameters were highly dependent on fuel cetane number.