The propensity of 5-selenocyanatouracil (SeCNU) to decomposition induced by attachment of electron was scrutinized with the G3B3 composite quantum-chemical method and radiolytic studies. Favorable thermodynamic (Gibbs free reaction energy of -13.65 kcal/mol) and kinetic (Gibbs free activation energy of 1.21 kcal/mol) characteristics revealed by the G3B3 free energy profile suggest SeCNU to be sensitive to electron attachment. The title compound was synthesized in the reaction between uracil and selenocyanogen chloride in acetic acid. Then, an aqueous and deoxygenated solution of the HPLC purified compound containing tert-butanol as a hydroxyl radical scavenger was irradiated with X-rays. SeCNU radio-degradation results in two major products: the U-Se-Se-U dimer and the adduct of the (center dot)OtBu radical to the U-Se-center dot radical, U-Se-OtBu. The effects of radiolysis as well as the results of G3B3 calculations point to U-Se-center dot as the primary product of dissociative electron attachment to SeCNU. The MTT test shows that SeCNU is nontoxic in vitro in concentrations equal to or lower than 10(-6) M. Ionizing radiation will probably induce cytotoxic intra- and interstrand DNA cross-links as well as protein-DNA cross-links in the genomic DNA labeled with SeCNU.