The anions [Se(S)CC(CN)₂]^2–(imnts) and [Se₂CC(CN)₂]^2–(imns) undergo oxidative dimerization reactions which have been studied by cyclic voltammetry. They proceed via a one-electron oxidation followed by a rapid chemical radical dimerization (EC_irr mechanism) with E_imnts=–0.2 V and E_imns=–0.3 V versus SCE. The resulting anionic diselenides have been characterized by X-ray structure analysis of [NBu₄]₂[{(NC)₂CC(S)Se–}₂]1, [AsPh₄]₂[{(NC)₂CC(S)Se–}₂]2 and [AsPh₄]₂[{(NC)₂CC(Se)Se–}₂]3. The anions in 1–3 all exhibit a trans arrangement with an inversion centre bisecting the Se–Se bridges. The average Se–Se bond length is 2.330 Å. There are two types of bonds involving sp²-hybridized carbon and different chalcogen atoms. The average Se–C bond distance involving selenium of the diselenide group is 1.936 Å and corresponds to a single bond. The chalcogen–carbon bonds involving anionic sulfur atoms of 1 and 2 as well as Se of 3 are 1.668, 1.669 and 1.81 Å respectively and seem indicative of some double-bond character with charge delocalization over the dimeric dianions.