Diesel fuel properties influence diesel exhaust emissions. This study determined the effect of fuel cetane number and aromatic content on emissions from a heavy-duty diesel engine meeting 1994 emissions standards. No exhaust aftertreatment devices were used in this study. The fuel set was selected to separate the cetane effects from the aromatics effects, since otherwise these effects can be easily confused. Other fuel property effects, such as density, were also considered in the selection of the fuel set.This study found that increasing cetane number reduces all regulated diesel emissions species. Reducing aromatic content reduces NOx and particulate emissions, as was found in the previous Amoco/Navistar study, but this time the aromatics effect was less and the cetane effect was greater. When compared to the CRC VE-1 Phase 2 study, aromatic effects were almost identical and cetane effects were similar on all emissions.An economic analysis of the test results indicates that the cost to reduce NOx through the reduction of diesel aromatics alone is about 10 times higher than the cost to achieve similar emission reductions by raising the fuel cetane number. The cost to reduce particulates is about 3 times higher through aromatics control alone than through cetane control. These results confirm that the EPA exhaust emissions standards for 1994 can be achieved without resorting to costly restrictions in diesel aromatic content, and that if additional fuel improvements are needed to meet post 1994 emissions standards, raising cetane number requirements will be far more cost-effective in controlling emissions than reducing aromatic content.Fuel regulations, especially those promulgated in California, will require costly changes in diesel fuel composition. These regulations are based on anticipated reductions in emissions due to fuel changes. However, both engine design and fuel set design can affect the observed response of exhaust emissions to fuel property changes. The results from this study using a 1994-type engine and a well-designed fuel set helps to update earlier models of fuel effects on heavy-duty exhaust emissions.