Abstract The cool flames and two-stage ignition of neopentane-oxygen mixtures have been studied, kinetically and analytically, to determine whether chain propagation in this oxidation régime occurs by isomerization and decomposition of alkylperoxy radicals and whether each lobe in the low-temperature ignition profile corresponds to a given mode of isomerization. Two lobes are formed, these being the lobes L 2 and L 3 of the general alkane-oxygen ignition profile. The products of the cool flames associated with both these lobes show that isomerization, by 1:5 H-transfer and 1:4 CH 3-transfer, is indeed a major mode of chain propagation. Although the properties of cool flames (induction period, intensity and rate of pressure rise) change suddenly at ca. 325°C, the natures of the chemical products of cool flames do not. It follows that a given mode of isomerization is not associated with a given lobe. Chain propagation in both lobes is similar; it is the branching process which changes from one lobe to another. Branching in the lobe L 2 is not due to the pyrolysis of neopentyl hydroperoxide; the oxidation of the one of the products of neopentylperoxy radical isomerization probably branches the rection in this régime.