A 260 kW DDC Series 60 heavy-duty diesel engine, calibrated for 1991-1993 emission standards, was used to generate transient emissions data at three fuel injection timings, using several fuels. The fuel matrix evaluated in this study consisted of a high cetane, low aromatic diesel fuel meeting CARB specifications for on-highway use, and a typical low-sulfur (nominally 0.04 wt.%) type 2D diesel served as the “base” fuel. The base fuel was then treated with the two different cetane improvers ethyl hexyl nitrate (EHN) and di-tertiary butyl peroxide (DTBP). Base fuel was also blended with soy methyl ester to yield a bio-diesel product. The bio-diesel was tested with and without the DTBP additive. Except for the type 2D base fuel, which had a cetane number of 45, most fuels tested in this study had a cetane number of 55, either naturally or improved. Emission tests were conducted at baseline timing, then at 3° retard, and finally at 5° retard. Particulate was characterized as carbon, ash, sulfate, and volatile organic fraction (VOF). The percent of VOF attributed to unburned lubricating oil was also quantified. The 2D base fuel resulted in the highest NOx and PM levels at each timing level tested in this study. Progressive timing retard reduced NOx and increased PM for all fuels tested, however smaller increases in PM were observed when using the low aromatic fuel and the bio-diesel product. The low aromatic fuel had the lowest NOx level at each timing, but bio-diesel had the lowest PM. At retarded timings, increases in total PM levels were attributed to unburned fuel derived VOF and carbonaceous material.