Comparable amounts of OH and HO2 radicals are trapped in hyperquenched glassy water gamma-irradiated at 77 K in a dose range up to about 20 kGy. Addition of thymine, in concentration as low as 0.005 M, changes their relative contributions, Thymine radicals (TH) are seen to be formed by addition of atomic hydrogen already at 77 K at the expense of the relative contribution of HO2 radicals. This is not due to the scavenging of once-formed HO2 radicals by thymine, but it occurs concurrently with HO2 radical formation. Therefore, it is rationalized by a concerted process involving the primary intermediate H2O+. Because of the large water-to-thymine ratio of approximate to 10(4) in homogeneous glassy solution, the radiation effect on thymine must be indirect. In contrast, it is known that DNA in freeze-concentrated solution is exposed to either direct or "water-mediated" radiation effects where TH radical formation occurs at much higher temperature.