The effect of diesel cetane number, total aromatic content T90, and fuel nitrogen content on regulated emissions (HC, CO, NOx, and PM) from a 1991 DDC Series 60 engine were measured Emissions tests were conducted using the EPA heavy-duty transient test (CFR 40 Part 86 Subpart N) at a laboratory located 5,280 feet (1609 m) above sea level. The objective of this work was to determine if the effect of fuel chemistry at high altitude is similar to what is observed at sea level and to examine the effect of specific fuel chemistry variables on emissions. An initial tea series was conducted to examine the effect of cetane number and aromatics. Transient emissions for this test series indicated much higher (50 to 75%) particulate emissions at high altitude than observed on the same model engine and similar fuels at sea level. The SOF fraction of the total PM appears to be significantly smaller at altitude (10-15% versus 20-35% at sea level) suggesting that the PM increase is from an increase in emissions of residual carbon. A second test series was conducted to determine the effect of high boiling point material (described by T90) on emissions. Two fuels having the same cetane number but with different T90, total aromatic, and PNA contents were used. Synthetic PNA as methyl naphthalenes was added to the lower aromatic fuel to bring its total aromatic and PNA content to the level of the second fuel. After this treatment the only major difference between the fuels was boiling point distribution. The fuel with 30°C higher T90 exhibited approximately 5% higher NOx and 30% higher particulate emissions. Plotting of all of the data in this paper on a NOx/PM trade off plot shows that fuels with higher boiling components shift the trade off to higher PM emissions. A third test series examined the effect of fuel nitrogen by addition of pyridine or dodecyl amine, and of cetane treating with ethylhexyl nitrate, on emissions. Both dopants produced less than a 1% increase in NOx emissions for increasing fuel nitrogen by a factor of 10. It was observed over a large number of transient tests that doping with pyridine caused a significant decrease in PM emissions, but this did not occur for dodecyl amine. Additional doping of these fuels with ethylhexyl nitrate resulted in a 2.5% NOx decrease but no effect on particulate relative to the base fuel.