The quenching rate of singlet oxygen (O-1(2)) by seven kinds of flavonoids (flavone, flavonol, chrysin, apigenin, rutin, quercetin, and myricetin) with 2,3-double bonds has been measured spectrophotometrically in ethanol at 35 degreesC. The overall rate constants k(Q) (=k(q) + k(r), physical quenching + chemical reaction) increased as the number of OH groups substituted to the flavone skeleton (that is, the total electron-donating capacity of flavonoids) increases. The existence of catechol or pyrogallol structure in the B-ring is essential for the O-1(2) quenching of flavonoids. Log k(Q) was found to correlate with their peak oxidation potentials, E-P; the flavonoids that have smaller E-P values show higher reactivities. Similarly, log k(Q) values of flavonoids correlate with the energy level of the highest occupied molecular orbital (E-HOMO), calculated by the PM3 MO method, and the longest wavelength pipi* excitation energy (E-ex). The contribution of the chemical reaction (k(r)) was found to be negligible in these flavonoids. The k(Q) values of rutin, quercetin, and myricetin [(1.21similar to5.12) x 10(8) M-1 s(-1)] were found to be larger than those of lipids [(0.9similar to6.4) x 10(4) M-1 s(-1)], amino acids (< 3.7 x 10(7) M-1 s(-1)),and DNA (5.1 x 10(5) M-1 s(-1)). The result suggests that these flavonoids may contribute to the protection of oxidative damage in foods and plants, by quenching O-1(2).