Regiospecifically chlorine-37-labeled polychlorodibenzo-p-dioxins (PCDDs) undergo electron energy-dependent regioselective chloride ion loss when analyzed by electron capture negative ion mass spectrometry using an electron monochromator to supply the slow monoenergetic electrons, Three negative ion resonances, produced with electrons of energies <0.5, similar to 1, and similar to 4 eV are associated with the production of chloride ions from PCDDs. Negative ion resonances for the production of molecular ions of higher PCDDs were recorded with electrons of energies <0.2 eV. Dichlorodibenzo-p-dioxins and some trichlordibenzo-p-dioxins showed no molecular ions. A correlation was found between the experimental electron attachment energies and the virtual orbital energies calculated by modified density functional theory methods using the B3LYP/D95// B3LYP/D95 level of theory. The results from these correlations strongly suggest that all negative ion-forming processes for this class of compounds are initiated from pi* states. The loss of chloride ion from transient negative PCDD ions requires pi*-sigma* orbital mixing.