An aqueous solution of thymine (2 x 10(-3) M, pH 5.6) was irradiated with 60Co gamma-rays in the absence and in the presence of CuSO4 (5 x 10(-4) M) under different conditions. In argon and N2O saturated solutions, the presence of CuSO4 increased the radiosensitivity of thymine to a greater extent, while the product distribution was changed. Thus in the presence of CuSO4 the formation of dimeric products was prevented and the yields of cis-thymine glycol and 5-hydroxymethyl uracil were greatly increased. The sensitized degradation was suggested to be due to the oxidation of the transient hydroxyl adducts of thymine by copper ions.