Reaction kinetics have been theoretically examined to ascertain the potency of quercetin (Q) and flavonoid catecholic metabolites1–5in the inactivation of HOO•, CH3OO•, and O2•−under physiological conditions. In lipidic media, thekoverallTST/Eckrate constants for the proton-coupled electron transfer (PCET) mechanism indicate the catecholic moiety ofQand1–5as the most important in HOO•and CH3OO•scavenging. 5-(3,4-Dihydroxyphenyl)-γ-valerolactone (1) and alphitonin (5) are the most potent scavengers of HOO•and CH3OO•, respectively. ThekoverallMfrate constants, representing actual behavior in aqueous media, revealQas more potent in the inactivation of HOO•and CH3OO•via single electron transfer (SET). SET from 3-O−phenoxide anion ofQ, a structural motif absent in1–5, represents the most contributing reaction path to overall activity. All studied polyphenolics have a potency of O2•−inactivation via a concerted two-proton–coupled electron transfer (2PCET) mechanism. The obtained results indicate that metabolites with notable radical scavenging potency, and more bioavailability than ingested flavonoids, may contribute to human health-promoting effects ascribed to parent molecules.