The gas phase and solution electron affinities and ionization potentials of uracil, thymine and a series of 5-halouracils (5XU, X = F, Cl, Br) are investigated with B3LYP. Halogen substitution has a smaller effect on the IP than the EA of U. The EAs are calculated to increase according to T < U < 5FU < 5CIU < 5BrU. The calculated barriers for the dissociation of the resulting 5XU anions to X- plus uracil-centered radicals decrease along the series 5FU(-) > 5CIU(-) > 5BrU(-). The calculated trends are consistent with suggestions that 5XUs enhance the sensitivity of deoxyribonucleic acid, DNA (ribonucleic acid, RNA) to ionizing radiation and that 5BrU leads to the greatest enhancement.