The major reactive species formed via reaction of hydroxyl radical and the pyrimidine nucleoside thymidine, 5,6-dihydro-5-hydroxythymid-6-yl (1), is generated photochemically under anaerobic conditions from 6 via photoinduced single electron transfer. Under the conditions in which it is generated, 1 is trapped by hydrogen atom donors to form thymidine C5-hydrate (7), and undergoes oxidation, resulting in the formation of thymidine glycol (8). Isotopic (H-2, O-18) labeling experiments indicate that dehydration of 1 is not competitive with intermolecular hydrogen atom donation by 3,3,6,6-tetradeuteriocyclohexa-1,4-diene. Extrapolation of the known rate constants for hydrogen atom donation by cyclohexa-1,4-diene to alkyl radicals suggests that intramolecular hydrogen atom abstraction and dehydration are <2 s(-1), and are not kinetically competent to be involved in nucleic acid strand scission that arises from 1. Relative quantum yields for disappearance of 6 in the presence and absence of cyclohexa-1,4-diene suggest that the diene reduces the N-methylcarbazole cation radical, preventing back electron transfer. Labeling studies using 3,3,6,6-tetradeuteriocyclohexa-1,4-diene and 1,2,3,4,5,6-hexadeuteriocyclohexa-1,4-diene suggest that the resulting olefin cation radical, or other reactive species derived from the trap, competes with cyclohexa-1,4-diene for 1.