The oxidation rates for conjugated linoleic acid (CLA), linoleic acid (LA), and their methyl and ethyl esters were measured in aqueous and solvent systems by the induction period method. In an aqueous system, the initiation rate was almost the same for all the samples, except for ethyl conjugated linoleate (ECL). The propagation rate, oxidizability, and kinetic chain length were different for the ester and free types of CLA and LA. CLA was the most stable, its oxidizability being half that of LA. However, ECL was very susceptible to free radical oxidation. In a solvent system, the CLA derivatives had higher or almost same oxidizability as the LA derivatives, although the propagation rates of CLA and LA were lower than those of their esters.