ESR spectra of the short-lived radicals of catechol and gallic acid formed by reaction with hydroxyl radical (HO.) in acidic solution were measured using a dielectric mixing resonator and were compared with those formed by autoxidation in an alkaline solution. The triple triplet and triplet absorptions of catechol and gallic acid in alkaline solution showed that they were the phenoxy radicals and that the residual phenolic hydroxyl groups (phi-OH) were ionized. On the contrary, the lines were broad and unresolvable in acidic solution, suggesting that the radicals were exchanging between some limiting structures with a rate that affected the line shape. The radical is thought to be phenoxy formed by dehydrogenation with HO., and the position of the unpaired electron can move to another phi-OH through a cationic radical structure as an intermediate. Thus, a simulation was performed assuming that the Bloch equation is applicable to some groups of the lines of the limiting structures. A method for the quantum-chemical analysis was developed, and the exchange rate and the hyperfine splitting constants were obtained from the best-fit spectra using a revised Marquardt method. The results suggest that polyphenols scavenge HO. using the phi-OHs, but it was impossible to determine which phi-OH was used from the ESR spectra because the structure was changing among a few structures.