E.s.r. techniques have been used to study the reactions of thymine, thymidine and dihydrothymine with electrons and O^â radical ions which are produced in the γ-irradiation of frozen aqueous solutions of sodium hydroxide. Electrons can attack thymine at the 5,6 double bond when after proton transfer the thymine radical is produced and an eight-line e.s.r. spectrum is observed. After annealing at temperatures above 77 K the O^â radical ion can react with the methyl group at the 5 position. In thymidine the eight-line e.s.r. spectrum is observed already at 77 K and this increases further in intensity on annealing. The O^â presumably reacts with the methyl group of dihydrothymine.