Abstract Antioxidants minimize the oxidation of lipid components in cell membranes by scavenging free radicals. However, imbalance between free radical production and removal tends to increase with age causing progressive damage. For the food industry it is of considerable interest to delay the autoxidation of food lipids, which cause the reduction in food quality, affecting color, taste, nutritive value, and functionality. A general orientation toward the use of natural compounds has stimulated research into the potential use of aromatic and medicinal plants as possible antioxidant replacements. This study characterized the antioxidant and pro-oxidant properties of thyme oil and a number of its components. The major components identified in thyme oil were found to inhibit ferric-ion-stimulated lipid peroxidation of rat brain homogenates, although none was as effective as the whole oil The order of antioxidant activity was; thyme oil > thymol > carvacrol > Î³-terpinene > myrcene > linalool > p-cymene > limonene > 1,8-cineole > Î±-pinene. Both thyme oil and thymol were also found to inhibit tert-butyl-hydroperoxide-stimulated peroxidation and INT reduction by superoxide radicals generated by the xanthine-xanthine oxidase system. Of these compounds tested only p-cymene, 1,8-cineole and myrcene were found to exhibit pro-oxidant activity, albeit to a very small extent. Overall, the data suggest that thyme oil possesses useful antioxidant properties that may be utilized in the food industry and as a dietary supplement.