The superoxide ion (O-2(.-)) has been generated electrochemically from oxygen dissolved in two different solvent systems: (1) acetonitrile with tetraethylammonium perchlorate (TEAP) as the supporting electrolyte at elevated pressure and (2) in a room-temperature ionic liquid, 1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim] [HFP]), at atmospheric pressure. A high-pressure electrochemical cell with a quasi reference electrode was developed for elevated pressure tests. Increasing the partial pressure of oxygen in the first system increased the rate of superoxide generation because of the increased solubility of oxygen according to Henry's law. The subsequent addition of gaseous carbon dioxide enhances the rate of oxygen reduction in both systems but inhibits the reverse (oxidation) reaction of O-2(.-) to O-2. This later observation is consistent with the irreversible formation of a peroxydicarbonate ion, as has been postulated by others.