In this work, addition of OH- to one-electron oxidized thymidine (dThd) and thyinine nucleotides in basic aqueous glasses is investigated. At pHs ca. 9-10 where the thymine base is largely deprotonated at N3, one-electron oxidation of the thymine base by Cl-2(center dot-) at ca. 155 K results in formation of a neutral thyminyl radical, T(-H).. Assignment to T(-H). is confirmed by employing N-15 substituted S'TMP. At pH >= ca. 11.5, formation of the 5-hydroxythymin-6yl radical, T(5OH)., is identified as a metastable intermediate produced by OH- addition to T(-H). at CS at ca. 155 K. Upon further annealing to ca. 170 K, T(50H)" readily converts to the 6-hydroxythymin-S-yl radical, T(6OH). Oneelectron oxidation of N3-methyl-thymidine (N-3-Me-dThd) by Cl-2(center dot-) at ca. 155 K produces the cation radical (N3-Me-dThd(center dot+)) for which we find a pH dependent competition between deprotonation from the methyl group at CS and addition of OH- to CS. At pH 7, the 5-methyl deprotonated species is found; however, at pH ca. 9, N3-Me-dThd(center dot+) produces T(SOH). that on annealing up to 180 K forms T(6OH).. Through use of deuterium substitution at C5' and on the thymine base, that is, specifically employing [51,5 ''-D,D]-5'-dThd, [5',5 ''-D,D]-5'-TMP, [CD3]-dThd and [CD3,6131-dThd, we find unequivocal evidence for T(50H). formation and its conversion to T(6OH).. The addition of OH- to the CS position in T(-H). and N-3-Me-dThd" is governed by spin and charge localization. DFT calculations predict that the conversion of the "reducing" T(5OH). to the "oxidizing" T(60H). occurs by a unimolecular OH group transfer from CS to C6 in the thymine base. The T(5OH)" to T(6OH). conversion is found to occur more readily for deprotonated dThd and its nucleotides than for N3-Me-dThd. In agreement, calculations predict that the deprotonated thymine base has a lower energy barrier (ca. 6 kcal/mol) for OH transfer than its corresponding N3-protonated thymine base (14 kcal/mol).