Gama (γ) radiolysis of N2O saturated baicalin aqueous solution was studied. Four major final products (P1, P2, P3, P4) were separated, and their structures were determined by UV, MS and NMR. It was confirmed that P1 and P2 came from OH radical addition to ring C, but P3 and P4 originated from the disproportionation reaction of OH adduct radicals of ring B. The rate constant, 2.6×10^9mol^-^1.dm^3.s^-^1 of OH radical addition to aglycone was derived.

Key words: ULTRAVIOLET SPECTROPHOTOMETRY, MASS SPECTROGRAPHY, NMR SPECTROMETRY, RADIOLYSIS, NITROGEN OXIDE, REACTION RATE CONSTANT