A study of cetane response of post-1993 U.S. No. 2 diesel fuels was performed following the October 1, 1993 introduction of the federally mandated 0.05% sulfur, on-highway diesel fuel. Seven fuels obtained from East Coast, Gulf Coast and Midwest refineries, an emissions certification fuel and a low aromatic California fuel were evaluated. Ethylhexyl nitrate was used as cetane improver in concentrations of up to 1 vol.%.Test results demonstrated relatively low cetane response of non-California fuels and an exceptionally high response of the low aromatic California fuel. The only predictive cetane response equation available in technical literature failed to accurately characterize the performance of fuels used in this study.An inter-industry program is proposed to (a) develop a detailed understanding of diesel fuel composition/property effects on cetane response and (b) define an improved cetane response equation applicable to the full range of diesel fuels.