The gas-phase oxidation of n-pentane-air mixtures in the high temperature pre-ignition region (500–620 °C) was studied at levels of conversion maintained at low values (0.5–6.1%) by using an annular flow reactor and very short residence times. Among the sixteen products identified and quantitatively monitored by gas chromatography and other methods were lower hydrocarbons, aldehydes, ketones and such cyclic ethers as 2-methyltetrahydrofuran, 2,4-dimethyloxetan and 2-ethyloxetan. Pentyl and peroxypentyl radicals were judged to be the first intermediates of the reaction and their pyrolysis to account for more than 40% of the n-pentane reacted at 520 °C and 70% at 620 °C. The contribution of the pentyl radical to this pyrolysis increased markedly with temperature. The mechanism proposed for the formation of the products is a relatively simple one involving a series of internal isomerization reactions of the peroxy and alkyl radicals, following a scheme put forward by Fish.