The use of Koopmans’ theorem (KT) for the calculation of scaled energies of pi* negative ion resonance states has been investigated by ab initio molecular theory as a function of basis set. HF/D95v energies calculated for HF/6-3 IG-optimized geometries have been found to correlate with experimental electron attachment energies for 56 pi* negative ion states in 39 alkenes, polyenes, and benzenoid hydrocarbons with a correlation coefficient of 0.989 and with average and largest absolute errors of 0.11 and 0.32 eV, respectively. An equally good correlation was obtained at the same basis set for MP2/6-31G*-optimized geometries. Various factors that influence the quality of KT correlations are analyzed. It is concluded that the D95v basis set can be routinely employed for the quantitative prediction of first and second pi* electron attachment energies in the classes of compounds investigated.