As a part of an overall project to improve the techniques for rating the ignition quality of diesel engine fuels, the experiments described in this paper involve examination of the relationship between cetane number and both the physical and chemical ignition delay times. The ignition delay times have been determined from accurate pressure histories obtained during the injection and ignition of a variety of test fuels in a constant volume combustion bomb using a quiescent, high-temperature, high-pressure air environment. The test fuels have included blends of the primary reference fuels as well as other fuels selected because of specific physical properties or chemical composition. The correlation between the cetane numbers of the fuels and various ignition delay times are examined. For petroleum derived fuels which have undergone traditional processing, the ignition delay times determined from the bomb experiments are highly correlated with the cetane numbers as determined in the cetane engine. The ignition delay times for the alternative and highly hydrotreated fuels do not show as good a correlation with cetane number as the other test fuels. These results are discussed in terms of the possible differences in physical properties or chemical composition.