The thermal oxidation of the renewable olefin β-pinene with molecular oxygen was experimentally and computationally investigated. Peroxyl radicals weakly bonded allylic hydrogen atoms from the substrate, yielding allylic hydroperoxides (i.e., myrtenyl and pinocarvyl hydroperoxide). In addition, peroxyl radicals add to the C=C bond of the substrate to form an epoxide. It was found that a relatively high peroxyl radical concentration, together with the high rate of peroxyl cross-reactions, make radical-radical reactions surprisingly important for this particular substrate. Approximately 60 % of these peroxyl cross-reactions lead to termination (radical destruction), keeping a radical chain length of approximately 4 at 10 % conversion. Numerical simulation of the reaction-based on the proposed reaction mechanism and known or predicted rate constants-demonstrate the importance of peroxyl cross-reactions for the formation of alkoxyl radicals, which are the precursor of alcohol and ketone products.