The two keto tautomers of phenol (1), cyclohexa-2,4-dienone (2) and cyclohexa-2,5-dienone (3), were generated by flash photolysis of appropriate precursors in aqueous solution, and the pH-rate profiles of their enolization reactions,2–>1and3–>1, were measured. The rates of the reverse reactions,1–>2and1–>3, were determined from the rates of acid-catalyzed hydron exchange at theortho-andpara-positions of1; the magnitude of the kinetic isotope effect was assessed by comparing the rates of hydrogenation of phenol-2tand -2d. The ratios of the enolization and ketonization rate constants provide the equilibrium constants of enolization, pKE(2, aq, 25°C) = -12.73 ± 0.12 and pKE(3, aq, 25°C) = -10.98 ± 0.15. Combination with the acidity constant of phenol also defines the acidity constants of2and3through a thermodynamic cycle. These ketones are remarkably strong carbon acids: pKa(2) = -2.89 ± 0.12 and pKa(3) = -1.14 ± 0.15. They disappear by proton transfer to the solvent with lifetimes, &tgr;(2) = 260 &mgr;s and &tgr;(3) = 13 ms, that are insensitive to pH in the range from 3-10.Key words: proton transfer, tautomers, flash photolysis, kinetic isotope effect, pH-rate profile