Several diesel fuel properties have been identified as having significant effects on diesel engine emissions. For heavy-duty diesel engines, fuel properties of aromatics, back end volatility (represented by the 90 percent boiling point), and sulfur were examined in a previous CRC VE-1 study in which reductions in all three properties decreased regulated emissions to varying degrees. Aromatic levels and cetane numbers were generally correlated in the previous study, so variation in emissions due to “aromatics” could not clearly be assigned to variation in aromatic levels alone. To separate the effects of aromatics and cetane number, a fuel set with controlled variation in aromatics and cetane number was developed, including the use of ignition improver to increase the cetane number of selected fuels.The fuel set was used in a 1991 Prototype DDC Series 60 heavy-duty diesel engine to examine regulated emissions over EPA transient cycle operation. Results indicate that cetane number was the key fuel property affecting transient HC and CO emissions. In addition, cetane number was the principal fuel property affecting composite particulate emissions, but aromatic effects were also significant. For emissions of NOx, both cetane number and aromatics were significant for transient emissions.