The potential energy surface (PES) relevant to the isomerization Of (CO2)(2)(-) has been investigated by ab initio calculations including the effects of electron correlation. The surface plotted against appropriate angle parameters shows two shallow minima and a deep valley, which correspond to the CO2-. CO2 (C-2v), CO2-. CO2 (C-s) and C2O4- (D-2d) structures, respectively. The transition states for the C-2v <----> C-s <----> D-2d interconversion are located well below the CO2- + CO2 dissociation limit, which allows (CO2)(2)(-) with sufficient internal energies to fluctuate among the isomeric forms. The calculations have also revealed quantitatively the behavior of the excess charge flow during the CO2_. CO2 <----> C2O4- electronic isomerization.