The effects of cetane number and the cetane enhancing additives on diesel exhaust emissions were investigated on a single cylinder DI research engine. The engine used in this study incorporates the features of contemporary medium-to-heavy duty diesel engines and is tuned to US EPA 1994 emission standards. The engine experiments were run using the AVL 8-mode steady-state simulation of the U.S. EPA heavy-duty transient test procedure. The experimental fuels included diesel fuels obtained from different sources with various natural cetane ratings as well as a number of fuels blended by adding two cetane improvers into three base fuels. The two cetane improvers we used were a nitrate-type additive and a peroxide-type additive. Increasing the cetane number resulted in a general decrease in NOx emissions. Similar reductions in NOx emissions were observed with increasing cetane number for all the base fuels irrespective of the cetane improver used in the fuel. This result indicates that the nitrogen introduced by the nitrate-type cetane improver into the fuel does not contribute to NOx formation. The reduction in NOx emissions with increasing cetane number occurred mainly at low load conditions for all fuels. At high load conditions, NOx emissions changed slightly with cetane number. Results with a limited number of fuels showed a possible effect of total aromatic content on NOx emissions. These findings indicate that an increase in cetane number from 45 to 55 combined with a reduction in total aromatic content from 30% to 10% would most likely bring down NOx emissions by 5 to 10%. We observed an overall increase in particulate emissions with an increase in cetane number.