The implementation of stringent diesel engine emissions regulations is growing worldwide. The use of high cetane diesel fuels is one of the more cost effective options that can be used to reduce engine emissions.Both organic nitrates and peroxides are effective chemical cetane improvers. While, nitrate-based ignition improvers have enjoyed longstanding commercial use, peroxides have only found limited use. Peroxides are typically perceived as having detrimental effects on fuel stability. For most, but not all peroxides, this perception is correct. The poor fuel stability observed in fuels containing peroxides results from the formation of radical species via peroxide decomposition. A peroxide with sufficient stability to prevent radical formation will avoid fuel compatibility issues. A di-alkyl peroxide has been identified that has minimal impact on fuel quality. The thermal stability of this peroxide will be compared to that of ethylhexyl nitrate. Data on the decomposition of the peroxide and ethylhexyl nitrate in diesel fuel at elevated temperatures will be presented. The data presented includes rates and energies of thermal decomposition. Diesel fuel is subjected to elevated temperatures in the injection systems of modern diesel engines. The effect on cetane number of exposing the cetane improved fuels to elevated temperatures will be discussed.