Ab initio molecular orbital calculations have been performed at the 3-21G and 6-31+G(d)//3-21G levels in order to gain insight into the effects of hydration on electron affinities and ionization potentials of the DNA bases in base pairs. Both AT and GC neutral, anion, and cation radical base pairs were hydrated by four waters. Hydrogen bond lengths between the hydration layer and the base pairs varied significantly upon ionization of the systems, hydrogen bond rupture being occasionally observed. Furthermore, calculated hydration energies reveal the strongest solvent/base pair interactions to arise in the anion radicals while the weakest interactions are observed in the cation radicals. These variations were those expected from the increased electron density (basicity) of anion radicals and decreased electron density (acidity) of cation radicals over the neutral parent DNA bases. Koopmans ionization potentials calculated for each base in the hydrated,complexes suggest no effect of the first hydration layer on the order in IP (C > T >> A > G), although a significant increase from those found for the nonhydrated base pairs is observed. Primary hydration increases the adiabatic ionization potentials of the purines in bask pairs by an average of 0.3 eV, while solvent and stacking interactions are predicted to substantially decrease the adiabatic ionization potentials of the primarily hydrated purines. Adiabatic electron affinities of the pyrimidines in base pairs increased by ca. 0.4 eV as;a result of base pair primary hydration. Although this effect does not appear to be sufficient to stabilize the anions in the gas phase at the computational level employed here, correction constants obtained in other work from comparison of high-level calculations to experimental data yield positive electron affinities. Variation in the composition of the primary hydration layer of GC intensifies the competition between cytosine and thymine for electron capture. Full solvation of the primary hydrated base pairs is estimated to raise the adiabatic electron affinities of cytosine and thymine by about 2.8 eV from the gas-phase electron affinities of the individual bases. After ion formation, rearrangement of the primary hydration shell results in a 0.9-6.6 kcal stabilization.