When the ignition of a car is switched off, the engine may continue to idle erratically with intermittent firing strokes. This condition, which is known as 'run-on', can be caused by compression.ignition of the fuel/air mixture. Such a spontaneous ignition phenomenon should share a chemical mechanism with spontaneous ignition under other conditions. The influence of fuel quality on run-on can be described quite closely by the octane number of the fuel. A fuller description has been obtained by measuring and comparing the eftk:ts on the duration of run.on and on research octane number of thirty one organic additives, a blend of iso-octane and n-heptane [primary reference fuel (PRF) 92] being used as ba~ fuel. li is shown that the absolute efti~cts of such additives on the tendency to run-on of PRF 92 are in close agreement with the predictions of a widely held theory of the chemistry of chain-propagation during spontaneous ignition (the alkylperoxy radical rearrangement theory), The effects on the duration of run-on relative to tho~ on research octane number IRON) are in broad agreement with the theory, but the differences in fuel behaviour under running-on conditions and under knocking conditions are smaller than were expected. I t-Butylhydroperoxide No 2 2,2-Dimethyipropane No 3 n-Hexane No 4 2,2-Dimethylbutane No 5 Cyclohexane No 6 Methanol No 70xiran (ethylene oxide) No 8 Acetone No 9 Butanone No 10 Acetic acid No 11 Methyl acetate No 12 2,3-Dimethylbutanc Yes 13 2,3,4-Trimethylpentane Yes 14 Ethyl acetoacetate Yes 15 Diethyl malonate Yes 1 6 N-Methylaniline Yes 17 6-t-Butyl-2,4dimethylphenol Yes 18 2-Methylbutane Yes 19 2-Methylpentane Yes 20 3-Methylpentane Yes 21 Acetaldehyde Yes 22 Diethyl ether Yes 23 lsobutane Yes 24 Ethylbenzene Yes 25 Isopropylbenzcne Yes 26 3-Methylbut-l-ene Yes 27 3-Methylbutan-2-one Yes 28 lsopropyl acetate-Yes 29 Di-isopropyl ether Yes 30 r-Butyl isopropyl ether Yes 31 Methyl isopropyl ether Yes * abstn ~.-abstraction; stal~ = stabilized,