The vertical ionization energies of the four nucleotides have been computed. Geometries have been chosen to mimic orientations as they appear in B-DNA. The negative charge on the phosphate was neutralized by protonation, and also by the inclusion of counterions. Calculations have been performed with electron propagator methods (P3), Moller-Plesset second-order perturbation theory, and density functional theory to determine the nature of the orbitals associated with the highest lying ionization energies. Calculations at the MP2/6311 G(d,p)HP3/6-311G(d,p) level of theory yield vertical ionization energies for 5 '-dTMP 9.05 eV, for 5 ' dCMP 8.40 eV, for 5 '-dAMP 8.16 eV and for 5 '-dGMP 7.96 eV. In all cases the highest occupied molecular orbital resides on the base moieties.