There is current interest in the use of naturally occurring flavonoids as antioxidants for the preservation of foods and the prevention of diseases such as atherosclerosis and cancers. To establish the molecular characteristics required for maximum antioxidant activity, electron spin resonance (ESR) spectroscopy has been used to determine the stoichiometry and kinetics of the hydrogen-donating ability of 15 flavonoids and d-alpha-tocopherol to galvinoxyl, a resonance-stabilized, sterically protected aryloxyl radical. The second-order reaction rates, which will be governed by O-H bond dissociation energies, were myricetin > morin > quercetin > fisetin approximately catechin > kaempferol approximately luteolin > rutin > d-alpha-tocopherol > taxifolin > tamarixetin > myricetin 3',4',5'-trimethyl ether > datiscetin > galangin > hesperitin approximately apigenin. Reactivity is highly dependant on the configuration of OH groups on the flavonoid B and C rings, there being little contribution from the A ring to antioxidant effectiveness. Highest reaction rates and stoichiometries were observed with flavonols capable of being oxidized to orthoquinones or extended paraquinones. However, rates and stoichiometries did not always correlate and the data suggest that kinetic factors may be of greater importance within a biological context.