The energetics of the dehalogenation of adenine-halouracil base pairs (A5XU), upon attachment of low-energy electrons, was investigated by use of density functional theory. These results are compared to those of single halouracils reported previously [J. Phys. Chem. A 2002, 106, 11248-11253]. Using the B3LYP functionals it was found that the gas phase adiabatic electron affinities (EA) of halogenated base pairs (A5BrU 0.59, A5ClU 0.56, A5FU 0.47 eV) are higher than that of AU (0.32 eV) and are slightly higher or comparable to the other DNA abundant base pair, guanine-cytosine (0.49 eV). Base pairing with adenine slightly decreases the EA of the halouracils, in contrast to the substantial increase in EA on base pairing of natural bases; as a result, the probability of electron capture by halouracils when in double-stranded DNA is suggested to be substantially reduced relative to that in single-stranded DNA. Even though the activation barriers for dehalogenation are small for both BrU-A and ClU-A, only the former has negative values of both DeltaH (-0.95 kcal/mol) and DeltaG (-1.52), while the latter has negative DeltaG (-0.28) but positive DeltaH (1.27). Infinite separations into halogen anions plus the remaining A-U-5-yl neutral radical are energetically unfavorable owing to sizable halide ion, radical interactions as reported earlier for non base paired halouracils. It is found that base pairing does not change the reactive nature of the uracil-5-yl radical. The results suggest that the radiosensitization properties of halouracils should be less effective in double-stranded DNA than in single-stranded DNA.