The electrochemical reduction of oxygen was investigated in some hydrophobic room-temperature molten salt systems (ionic liquids) consisting of bis(trifluoromethanesulfone) imide (TFSI-) anion with trimethyl-n-hexylammonium (TMHA(+)), 1-butyl-1-methylpyrrolidinium (BMP+), 1-ethyl-3-methylimidazolium (EMI+) or 1,2-dimethyl-3-propylimidazolium (DMPI+) cation. The oxygen dissolved in these melts was reduced to the superoxide ion on a gold electrode. The superoxide ion was stable against the aliphatic and alicyclic organic cations (TMHA(+) and BMP+) but reacted with the aromatic ones (EMI+ and DMPI+), suggesting that nucleophilic reagents attack the melts consisting of imidazolium cations. The acceptor number of these TFSI--based molten salt systems was estimated to be comparable to those of acetonitrile and dimethyl sulfoxide from the comparison of the redox potential of O-2/O-2(-). The superoxide ion was further reduced, presumably to the peroxide, on a gold electrode in the melts consisting of the aliphatic or alicyclic organic cation. (C) 2003 The Electrochemical Society.