Di-tert-butyl peroxide is a well-known accelerant for spontaneous ignition of practical fuels in a diesel engine. We have found, however, that it retards the spontaneous ignition of pure hydrocarbon fuels in an engine under simulated cold-starting conditions. We report on modeling calculations which suggest why di-tert-butyl peroxide could inhibit the spontaneous ignition of n-hexadecane, and on experiments designed to elucidate the mechanisms by which this peroxide works as an ignition improver in commercial fuels. It appears that di-tert-butyl peroxide neutralizes the inhibiting effects of some naturally present amines in the hut fuel spray droplets. (C) 2000 by The Combustion Institute.