In this density functional theory investigation of the radiosensitization properties of 5-halogen-substituted uracils, the potential energy surfaces of the halouracils before and after electron attachment are investigated. The electron affinities (EA's) of uracil, halouracils, and uracilyl radical (U-yl^•) are calculated. The gas-phase adiabatic EA's of the halouracils after zero point energy (ZPE) corrections are in good agreement with those reported recently (Wetmore, S. D.; Boyd, R. J.; Eriksson, L. A. Chem. Phys. Lett. 2001, 343, 151−158). The U-yl^• radical has an exceptionally high AEA of 2.34 eV and proton affinity of 9.5 eV in the gas phase, showing its reactive nature and potential to cause DNA damage when incorporated in the genome. The higher EA of the halouracils compared to that of the DNA bases supports the experimental reports on the increased probability of low-energy electrons to localize on halouracils in DNA, leading to dehalogenation reactions and DNA damage. Potential energy surfaces (PES) are calculated for dehalogenation to show the relative energy change in the dissociation of halogen from both the neutral molecule and anion radical. The PESs along the C₅−X bond of all neutral molecules including uracil show the typical surface expected for a strong covalent bond rupture. Each of the halouracil anion radicals is found to have two thermally accessible electronic states of differing symmetries, i.e., π*(A") and σ*(A^’), that have quite differing properties. Both the pure π* state and the σ* state feature planar geometries. The pure π* state has a PES similar to that of the neutral molecule with a strong C−X bond, while the σ* state shows far weaker C−X bonding. Moreover, there is a mixed state PES that undergoes a transition from a slightly nonplanar π* state to that of a σ* state as the C−X bond distance increases to the crossing point of the two PES. From the full PES that allows for state crossing, the lowest energy barriers for formation of the extended σ* states are estimated to be 20.80, 3.99, and 1.88 kcal/mol for F-, Cl-, and Br-substituted uracil anion radicals, respectively. The overall energetics suggest that the π* to σ* conversions are exothermic for ClU and BrU anions, with ΔH calculated to be −0.98 and −2.98 kcal/mol, ΔG, −2.32 and −3.80 kcal/mol at 298 K and 1 atm, respectively. Remarkably, for the F−U anion the lowest energy path is not the loss of fluoride ion but the detachment of HF. The sensitivity of the halouracils to low-energy electrons is found to be on the order of BrU ≈ ClU ≫ FU, in agreement with experimental observations.