Abstract Steady-state γ radiolysis of oxygen-free aqueous solution of thymidine has been carried out in the presence of cysteine and five aromatic sulfydryl and disulfide agents. The common major mode of action of these compounds is their ability to scavenge water radiolysis species including OH • radicals and solvated electrons. In addition, cysteine and the two compounds with a free SH group were found to act as hydrogen donor leading to an increase in the formation of the 5R and 5S diastereoisomers of 5,6-dihydrothymidine and 5-hydroxy-5,6-dihydrothymidine. Two sulfhydryl agents including cysteine were also found to chemically repair the radiation-induced osidic radicals through efficient hydrogen donation mechanisms. It should be noticed that hydrogen atom transfer to hydroxyl and electron thymidine adducts leads to the decomposition of the thymine moiety.