The carbonate radical anion CO3•− is a decomposition product of nitrosoperoxycarbonate derived from the combination of carbon dioxide and peroxynitrite, an important biological byproduct of the inflammatory response. The selective oxidation of guanine in DNA by CO3•− radicals is known to yield spiroiminodihydantoin (Sp), guanidinohydantoin (Gh), and a novel intrastrand cross-linked product, 5’-d(CCATCG*CT*ACC) between guanine C8 (G*) and thymine N3 (T*) atoms in the oligonucleotide (Crean et al., Nucleic Acids Res., 2008, 36, 742–755). Involvement of the T-N3 (pKa of N3-H is 9.67) suggests that the formation of 5’-d(CCATCG*CT*ACC) might be pH – dependent. This hypothesis was tested generating CO3•− radicals by the photodissociation of carbonatotetramminecobalt(III) complexes by steady-state UV irradiation that allowed for studies of product yields in the pH 5.0 – 10.0 range. The yields of 5’-d(CCATCG*CT*ACC) is ~ 45 times greater at pH 10.0 than at pH 5.0, which is consistent with the proposed mechanism that requires N3(H) thymine proton dissociation followed by nucleophilic addition to the C8 guanine radical.