The electron affinity (EA) of the most toxic dioxin, 2,3,7,8-tetrachlorinated dibenzo-p-dioxin (TCDD), has been obtained along with that of two other toxic TCDDs using density functional theory (DFT) for the first time. The vertical electron affinity calculated for 1,2,3,4- TCDD at the B3LYP/aug'-cc-pVDZ level agrees with the recently obtained (J. Am. Chern. Sec. 1999, 121, 2561) experimental electron energy, which produced a maximum anion intensity in electron-capture negative-ion mass spectrometry. The adiabatic EAs of 0.259 and 0.265 eV for 2,3,7,8-TCDD obtained using B3LYP and B3PW91 functionals, respectively, suggest that anionic TCDDs are stable with respect to the detachment of electrons and indicate that TCDDs act as electron accepters in the reaction with receptors in living cells. Because of the large difference in geometry between the neutral and the anionic 1,2,3,4-TCDD, the calculated adiabatic EA differs considerably from the vertical electron affinity. Anionic 1,3,6,8- and 2,3,7,8-TCDD are planar like their neutral counterparts; on the other hand, anionic 1,2,3,4-TCDD is nonplanar although its neutral counterpart is planar.