H-1 NMR and EPR spectroscopies were employed to detect the evolution of lipid peroxidation products resulting from thermal stress in a vegetable oil. The obtained concentration profiles indicate that the secondary oxidation products (saturated and unsaturated aldehydes) may form not only via a direct degradation of primary oxidation products (hydroperoxides), as assumed in the classic kinetic models. In order to explain the observed concentration profiles, an alternate kinetic model is proposed where the aldehydes are additionally generated from hydroperoxides through an independent pathway.