Studies of protolytic equilibria and investigations of stability of flavonoids at different acidities are necessary to better understand their antioxidant efficiencies and autoxidation characteristics. The protonation constant of carbonyl group and dissociation constants of OH groups of quercetin in aqueous solutions were determined spectrophotometrically. The distribution diagram of ionic species in aqueous solutions of various acidities was calculated. Study of the effects of UV irradiation on quercetin at pH 5.00, 7.50 and 10.00 indicated that UV irradiation accelerated quercetin autoxidation via the formation of the oxidation product. The stability of quercetin and oxidation product was investigated as a function of irradiation time by using spectrophotometric and HPLC techniques. The apparent pseudofirst-order rate constants for quercetin degradation and oxidation product formation were calculated and discussed.