The goal of this work was to investigate the ignition characteristics of several fuels and to try to determine why the cetane number accurately predicted ignition quality of some fuels while failing for others. The measurements were made under simulated diesel engine conditions in a constant-volume combustion bomb. The fuels were some of the same fuels tested in DI and IDI diesel engines by Needham and Doyle of Ricardo. Blends of the reference fuels used for cetane number rating were also tested for comparison.The results show that the cetane number, as currently defined, cannot provide a consistent and accurate measure of the ignition quality of fuels whose ignition characteristics depend on temperature (i.e., compression ratio) in a different manner than the reference fuels. An implication of this result is that the cold-start characteristics of a fuel cannot be determined from its cetane number if its ignition characteristics are not modeled by the reference fuels.