A complex phenomenology of auto-ignition, similar to II-alkanes and n-alkenes, has been revealed between 600-900 K and at pressures above 14 bar by studying in a rapid compression machine stoichiometric mixtures of o-xylene (o-methyltoluene), o-ethyltoluene, or n-butylbenzene in oxygen with lower concentrations than in air. Extensive chemical analyses of the reacting mixtures before ignition were performed to elucidate the mechanisms of reaction. The classical low temperature scheme, modified for the reactivities of benzylic-type hydrogen atoms and radicals, is valid. It appears that the addition of molecular oxygen to benzylic-type radicals leads to a double peroxidation and low temperature branching only when the transfer of hydrogen in the isomerization step occurs either from an ortho-alkyl group, or from another carbon atom of the same alkyl chain. The products observed are shown to be consistent with the proposed mechanism. The same complex pattern of auto-ignition is found, not only for o-xylene, o-ethyltoluene, and n-butylbenzene, but also for 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, n-propylbenzene, and ethylbenzene. When easily transferable hydrogen atoms are not available for selective radicals such as peroxy radicals, branching occurs through completely different pathways, which require higher temperatures and pressures. Then, the pattern of auto-ignition is much simpler, as already observed for toluene, m-xylene, p-xylene, and 1,3,5-trimethylbenzene [1],