Density functional theory calculations were performed to obtain the structures, vertical electron affinities, and adiabatic affinities of 15 polychlorinated dibenzo-p-dioxins (PCDDs), including several extremely toxic congeners. A three-parameter hybrid density functional, B3LYP, was utilized with two different basis sets, 6-311 G(d,p) and 6-311 +G(2d,2p). The optimized structures of all PCDDs under consideration were planar, while all corresponding anions attained nonplanar geometries. One of the C-Cl bonds on each PCDD anion was considerably elongated, and the dechlorination of PCDDs occurred as the departing chlorine bent off the aromatic ring plane for effective pi-sigma orbital mixing. The characteristic electron energy-dependent regioselective chloride ion loss channels for 1,2,3,7,8-pentaCDD were elucidated by transition-state theory calculations. The relative low-energy barrier for the dechlorination of I,2,3,7,8-pentaCDD indicated the high likelihood of obtaining reductive dechlorination (RD) products that are more toxic than the parent species. The calculated vertical electron affinities of PCDDs are consistent with the available experimental attachment energies, and the positive adiabatic electron affinities suggest that PCDDs may act as electron acceptors in living cells.