The low-temperature oxidation of n-heptane and iso-octane in mixtures with air in a jet-stirred-how reactor have been compared under suitable high-pressure conditions, such that the two mixtures of hydrocarbon and air showed comparable fuel conversions and phenomenologies. The large presence of aldehydes in the products of the low-temperature oxidation of n-heptane was attributed to a degenerate chain-branching path involving the addition of molecular oxygen to heptylhydroperoxy radicals and isomerization by internal H-atom abstraction. The latter step is particularly favored in linear alkanes where easy-to-abstract H-atoms are available. On the other hand, cyclic ethers and fuel-conjugate olefins were the dominant products of the low-temperature oxidation of iso-octane. This is due to a lack of H-atoms for internal abstraction; this limits the degenerate chain-branching route and favors the propagation path toward species having the same skeleton of the fuel, such as cyclic ethers and fuel-conjugate olefins. The prevalence of a degenerate chain-branching path for n-heptane compared with the propagation reactions in iso-octane oxidation is responsible for the different autoignition tendencies of n-heptane and iso-octane.