Radiation-induced and photosensitized one-electron oxidation of stereoisomeric CS-CS'-linked dihydrothymine dimers (1a,b[meso], meso compound of (5R,5'S)-bi-5,6-dihydrothymine; la,b[rac], racemic compound of (5R,5'R)- and (5S,5'S)-bi-5,6-dihydrothymines), which are the major products yielded by radiolytic reduction of 1-methylthymine (2a) and 1,3-dimethylthymine (2b) in aqueous solution, was studied to compare with the photoreactivating repair mechanism of cyclobutane pyrimidine photodimers. Reacting with sulfate radical anion (SO4.-), azide radical (N-3(.)), Or photoexcited anthraquinone-2-sulfonate (AQS) as oxidants, the C5-C5'-linked dihydrothymine dimers la,b split to regenerate the corresponding thymine monomers 2a,b along with 5,6-dihydrothymines (3a,b) in a pH dependent manner. The transient absorption spectra of 5,6-dihydrothymin-5-yl radicals (6a,b) were observed in the nanosecond laser flash photolysis of 1a,b in phosphate buffer under conditions of SO4.- generation. Both the product study and the laser flash photolysis study indicated an oxidative splitting mechanism by which one-electron oxidation of the C5-C5'-linked dimers la,b produces the radical cation intermediates (4a,b), which undergo facile fragmentation into 5,6-dihydrothymin-5-yl radicals 6a,b and CS-cations (5a,b), followed by deprotonation at C6 of 5a,b to regenerate the monomers 2a,b.