By means of the stopped-flow-ESR and -optical methods, oxidation of quercetin by nitrsodisulfonate (NDS) radical anion was studied. The results of optical measurements demonstrated that NDS readily oxidize quercetin to the quinone form. The rapid field-sweep ESR spectrum revealed the formation of short-lived quercetin-derived o-semiquinone anion radical. The radical was confirmed to be a transient intermediate species for generation of the quinone of form.