The antioxidant properties of three related compounds, dehydrozingerone, isoeugenol and eugenol, were investigated using various models. Isoeugenol was found to be the most active in inhibiting ferrous-ion-, ferric-ion- and cumene-hydroperoxide-induced lipid peroxidation in rat brain homogenates. These compounds also showed significant hydroxyl radical scavenging activity. Isoeugenol was potent in scavenging superoxide anion generated by the xanthine-xanthine oxidase system, whereas eugenol was found to inhibit xanthine oxidase. The high antioxidant activity of isoeugenol may be due to the presence of a conjugated double bond, which increases the stability of the phenoxyl radical by electron delocalization. Such electron delocalization is not possible with eugenol. In dehydrozingerone, the stability was decreased by an electron withdrawing keto group at the para position.