The goal of this work was to better understand the relationship between diesel fuel composition and its ignition performance. Ignition delay measurements were made as a function of temperature in a constant-volume combustion bomb at simulated diesel engine conditions. The fuels studied were binary mixtures of pure compounds and for comparison Phillips Diesel Control Fuel. The fuels were tested with and without cetane improver additive.The results show that the mechanisms of fuel autoignition change with temperature and composition. Change points correspond well to the low-, intermediate-, and high-temperature regimes defined in classical hydrocarbon oxidation studies. Differences in ignition performance are discussed in terms of the production of effectively chain terminating stabilized free radicals.Cetane number improver additive enhanced the autoignition performance of all fuels. The amount of improvement vs. unadditized fuel was related to both the fuel composition and temperature. These results appear consistent with the hypothesis that cetane number improver additive functions as a source of free radicals which most helps fuels that form stabilized intermediates at the expense of reactive free radicals.