The purpose of this study was to evaluate the relationship between solution-phase thermal stabilities of selected fuel additives and their effectiveness as combustion improvers. The additives selected for this study were 2-ethylhexyl nitrate, isopropyl nitrate, tetraethylene glycol dinitrate, di(tert-butyl) peroxide, and methylcyclopentadienyl manganese tricarbonyl. Decomposition studies were carried out at various temperatures on the neat additives as well as the additives dissolved in various solvents and fuels. Differential scanning calorimetry was used to survey thermal stabilities; high-pressure differential scanning calorimetry was used to determine the effects of pressure on the decomposition exotherms of the additives. Critical temperatures were calculated by the Frank-Kamenetskii equation for each additive. Neither the temperature of maximum exotherm nor the heat released by additive decomposition correlated well with the effectiveness of a given additive as a cetane improver.