In order to see the accurate role of low temperature reactions in autoignition nd nock phenomena, oxidation of stoichiometric hydrocarbons/air mixtures has been performed in a CFR engine and in a flow system. The results show that reactions of isomerization of R02 radicals areimponant for hydrocarbons containing five or more carbon atoms. The pernxidic compounds formed through these isomerization reactions are alkylketohydroperoxides. A correlation is made between fuel structure, octane number. isomerization reactions and autoignition. A chemical kinetic model. created to interpret autoignition and knock. reproduces the results obtained with n-heptane and the influence of engine parameters as speed, intake temperature and pressure.