It is demonstrated that the negative highest occupied molecular orbital eigenvalues of a molecule S in the gas phase, -HOMO(S,g), and in solution, -HOMO (S,s), and the negative lowest unoccupied molecular orbital eigenvalue of its cation S+ in solution, -LUMO(S+,s), are good approximations to the oxidation energy, the energy for removing an electron out of a molecule in solution. This observation is based on the DFT calculations using the B3LYP exchange correlation functional with a basis set of 6-311+G(d,p) and the polarizable continuum model (PCM) for computing solvation energies for a set of 22 middle-size molecules/radicals.