A study of how fuel properties and chemistry affect regulated emissions was conducted on two heavy-duty diesel engines which demonstrated emissions below the 1994 legal limits. The seven fuels used were specially blended so that no strong correlation existed between key fuel parameters. All of the fuels contained less than .06 wt.% sulfur. Gaseous and particulate emissions from transient testing were evaluated. Steady state tests were conducted at 24 operating modes and 3 injection timings. Heat release and gaseous emissions analyses were conducted for the steady state tests. Statistical methods were used to evaluate the fuel effects on emissions. It was determined that aromatic content is the primary fuel parameter driving NOx and particulate emissions for engines operating below 1994 emission levels. The relative reduction of emission levels due to changes in fuel composition was found to be the same for engines operating below 1994 legal limits as for engines operating at 1988 and 1991 legal limits. Finally, the relationships between fuel composition, heat release characteristics and emission responses were statistically demonstrated.