The reactivity of flavonoids as radical scavengers was investigated under kinetic considerations using radical polymerization of methyl methacrylates initiated by benzoyl peroxide. The number of radicals which are trapped by each molecule of phenol (the stoichiometric factors, n values) decreased in the order of epigallocatechin-3-O-gallate (ECG) (5.5) > catechin (3.5) > resveratrol (2.4) > quercetin (1.9) > n-propylgallate (1.5) > hesperetin (1.0). The inhibition rate constants (k(inh)) (1-3 x 10(3) 1/(mol s)) for the flavonoids were not different from each other, and, therefore, the radical scavenging activity depend on n values. The n values of the fully oxidized flavonoids were estimated from the frontier orbital theory, using PM3 semiempirical molecular orbital calculation. The experimental n values were consistent with the calculated values.