Recently, a phenylselenyl-modified thymidine (2) was shown to produce DNA interstrand cross-links (ICLs) via two mechanisms. Photolysis of 2 generates 5-(2'-deoxyuridinyl)methyl radical (1), the reactive intermediate that results from formal hydrogen atom abstraction from the thymine methyl group. This reactive intermediate reacts with the opposing dA and is the first example of a DNA radical that produces ICLs. Kinetic competition studies support the proposal that the rate-limiting stop in ICL formation from 1 involves rotation about the glycosidic bond and that the rate constant for this process is influenced by the flanking sequence. Cross-links also form with the opposing dA when 2 is treated with mild oxidants that result in the formation of an intermediate methide-like species (4). Kinetic experiments reveal that 4 reacts with azide, a model nucleophile, via an S(N)2' pathway. Previous experiments suggested that the same product is produced via 1 or 4 but that the initially formed cross-link rearranges during the enzyme digestion and isolation procedures. In situ product analysis by NMR using synthetic, doubly labeled duplex DNA containing C-13-2 and N-15(1)-dA provides definitive evidence that the kinetic ICL products formed via the radical and oxidative pathways are the same and correspond to that arising from formal alkylation of N-1-dA. Furthermore, analysis of the thermodynamic product formed upon rearrangement indicates that the primary product isomerizes via an associative mechanism in DNA.