Four commercially available low-sulfur diesel fuels were additized with two chemically different cetane improvers. Both neat and additized fuels were evaluated in a 1991 prototype heavy-duty diesel (HDD) engine using the EPA Hot Start Transient Cycle. Hydrocarbon (HC), carbon monoxide (CO), nitrogen oxide (NOx), and particulate emissions were determined for each of the 18 fuel formulations tested. Results show that cetane improvers lower HC and CO emissions and, in some cases, NOx and particulate emissions. CO and HC emissions decreased as cetane number increased. The use of cetane improvers should help refiners design diesel fuel formulations which meet California requirements and assist original equipment manufacturers (OEM's) in meeting their emission targets.