5,6-Dihydrothymid-5-yl (4) is generated via Norrish type I cleavage of isopropyl ketone 7. Ketone 7 was site specifically incorporated into chemically synthesized polythymidylates and an oligonucleotide containing all four native deoxyribonucleotides. No damage is induced in oligonucleotides containing 7 upon photolysis under anaerobic conditions. In the presence of O-2, strand breaks and alkaline labile lesions are formed at the original site of 7, and at nucleotides adjacent to the 5’-phosphate of 7. Kinetic isotope effect experiments reveal that direct strand scission at the thymidine adjacent to the 5’-phosphate of 4 arises from C1’ hydrogen atom abstraction. The observed KIE (similar to 3.9) is attributed to hydrogen atom abstraction from C1’ by the peroxyl radical 35 derived from 4. Enzymatic end group analysis and measurement of free base release are consistent with a process involving C1’ hydrogen atom abstraction. Cleavage experiments carried out in the presence of t-BuOH (1.05 M) and NaN3 (10 mM) indicate that damage does not result from hydroxyl radical, but that O-1(2) is responsible for a significant amount of the observed strand damage.