The antioxidant eþ ect of the ýavonoids quercetin, myricetin, kaemferol, (+ )-catechin and rutin on methyl linoleate oxidation was investigated. In addition, the synergistic eþ ects of ýavonoids and a-tocopherol were studied. Oxidation was monitored by conjugated diene measurement and by determining the formation of hydroperoxide isomers by HPLC. The antioxidant activity of ýavonoids in non-polar methyl linoleate diþ er from that previously reported in water-containing systems, such as LDL and liposome systems. The activity of antioxidants (10-1000 lM) measured by hydroperoxide formation decreased in the order : myricetin > quercetin > a-tocopherol > (+ )-catechin > kaemferol = rutin. The antioxidant activity of ýavonoids increased as the number of phenolic hydroxyl groups increased. In addition to the number of hydroxyl groups, other structural features such as the 2,3 double bond in the C-ring and a glycoside moiety in the molecule had an eþ ect on the antioxidant activity. Myricetin and rutin, especially had a synergistic eþ ect with a-tocopherol. Myricetin, quercetin and rutin protected a-tocopherol from decomposition, myricetin being the most protective. The relative hydrogen-donating activity measured by the ratio of cis,trans-to trans,transhydroperoxide isomers formed during oxidation decreased in the order : a-tocopherol > myricetin > quercetin. Hydroperoxide isomeric distribution of the samples containing kaemferol or rutin did not diþ er from the control. Thus, although a-tocopherol was the most eþ ective hydrogendonor, myricetin and quercetin were more eþ ective antioxidants in inhibiting the hydroperoxide formation in methyl linoleate.