1. A method for synthesis of 5-S-cystamine-5,6-dihydrothymine was developed. 2. This compound is stable in neutral and acid solutions but rapidly decomposes according to a first-order reaction at pH 10–12. It was found that decomposition is due to acidbase dissociation of 5-S-cystamine-5,6-dihydrothymine (pK ∼ 11.1) and that the rate of decomposition increases with an increase in the pH of the solutions. 3. The products of cross-linking of these compounds, 5-S-cystamine-6-hydroxythymine and 5-S-cystamine-5,6-dihydrothymine, are formed in radiolysis of deaerated acid solutions containing thymine and cystamine.