DNA interstrand cross-links have important biological consequences and are useful biotechnology tools. Phenylselenyl substituted derivatives of thymidine (1) and 5-methyl-2'-deoxycytidine (5) produce interstrand cross-links in duplex DNA when oxidized by NaIO4. The mechanism involves a [2,3]-sigmatropic rearrangement of the respective selenoxides to the corresponding methide type intermediates, which ultimately produce the interstrand cross-links. Determination of the rate constants for the selenoxide rearrangements indicates that the rate-determining step for cross-linking is after methicle formation. Crosslinking by the thymidine derivative in duplex DNA shows a modest kinetic preference when flanked by pyrimidines as opposed to purines. In contrast, the rate constant for cross-link formation from 5 opposite dG in duplex DNA is strongly dependent upon the flanking sequence and, in general, is at least an order of magnitude slower than that for 1 in an otherwise identical sequence. Introduction of mispairs at the base pairs flanking 5 or substitution of the opposing dG by dI significantly increases the rate constant and yield for cross-linking, indicating that stronger hydrogen bonding between the methide derived from it and dG compared to CIA and the respective electrophile derived from 1 limits reaction by increasing the barrier to rotation into the required syn-conformation. Incorporation of 1 or 5 in triplex forming oligonucleotides (TFOs) that utilize Hoogsteen base pairing also yields interstrand cross-links. The dC derivative produces ICLs approximately 10x faster than the thymidine derivative when incorporated at the 5'-termini of the TFOs and higher yields when incorporated at internal sites. The slower, less efficient ICL formation emanating from 1 is attributed to reaction at N1-dA, which requires local melting of the duplex. In contrast, 5 produces cross-links by reacting with N7-dG. The cross-linking reactions of 1 and 5 illustrate the versatility and utility of these molecules as mechanistic probes and tools for biotechnology.