[en] The mechanism and kinetics of glycyrrhizic acid monoamonium (GAM) reacting with hydroxyl radical generated by γ-ray irradiated N₂O-saturated aqueous solutions were studied. The radiolytic products were separated with HPLC and the molecular structures were determined by UV, IR, MS and elementary analysis methods. The major product was formed by the attack of hydroxyl radical to the C-C double bond of GAM. The others resulted from the release of one and two glucuronic acid molecules in GAM respectively. The rate constant of GAM reacting with the hydroxyl radical was measured using a competition method by pulse radiolysis technique to be of 3.0x10⁹ dm³ mol^-1 s^-1. The transient species generated by pulse radiolysis of GAM aqueous solution saturated with N₂O at pH 10 exhibited an absorption maximum at about 325 nm with extinction coefficient 260 dm³ mol^-1 cm^-1. The rate constant of second-order decay of transient species was 5.8 x 10⁸ dm³ mol^-1 s^-1. The transient species was formed by the attack of hydroxyl radical to the C-C double bond of GAM. The G values of GAM and its radiolytic products of 4.0x10^-4 mol dm^-3 aqueous solution saturated with N₂O at absorbed dose of 500 Gy are: G(-GAM) = 2.4, G(RP1) 2.2, G(RP2) = 0.3, G(RP3) = 0.1