At the first stage of electrochemical reduction in DMF, fluorinated/chlorinated 2,1,3-benzothia/selenadiazoles formed long-lived radical anions characterized by EPR and DFT. Gas-phase electron affinities (EA1) from DFT correlated well with the first-peak potentials separately for S and Se derivatives, and the latter were found to be better electron acceptors than the former in contrast to the atomic EA1 and Allen electronegativity. At the second stage, chalcogen-and halogen-dependent dehalogenation proceeded: non-hydrodefluorination of selenadiazoles through their n-electron activation (n ≥ 2), and hydrodechlorination of thia/selenadiazoles through H+ addition to their dianions. These differ from dehalogenation of related (aza) aromatics (e.g. benzenes, naphthalenes, quinoxalines).