During the last two decades, an appreciable number of papers have been devoted to the study of the superoxide ion 02. This radical anion possesses a 2Hg ground state, corresponding to a (Hg2p) 3 configuration t. Its interatomic distance has been evaluated 2 as 1.28 N. There is no definite evidence for the existence of the free tetroxide dimer 04 z-, which possibility was explored recently in the belief that this dianion could be stabilized by the same process as that which occurs with dinitrogen tetroxide or the tetrasulfides 2. 02 is a known species in the gas phase, in liquid ammonia and in various molten salts. In aprotic solvents, it is a long-lived species, easily generated by electroreduction of Oz on a variety of electrodes. The reaction O2 + e~-~--02 is usually reversible or quasi-reversible {i.e. k0 > 10-3 cm s-1) and seems to proceed faster on mercury than on noble metals such as platinum. Typical values of the polarographic half-wave potentials range from -0.73 V to -0.89 V (vs. aqueous SCE) according to the nature of the solvent. The scattering, which is remarkably small, is probably due to variations in solvation, diffusion coefficients and in the values of the junction potential between the solution and the reference electrode. That 02 may interact with high-field cations such as Li รท, Mg 2 +, Ca 2 +, Sr 2 + to give insoluble salts is indicated by a positive shift of Eยฝ, which may exceed one decivolt 3. 02 may be further reduced to the peroxide state. This process is highly irreversible and its rate constant is markedly affected by the nature of the supporting cation. The "bulk" stability of O~ depends on its reactivity with respect to the solvent and traces of contaminant, or toward itself (the disproportionation mechanism being enhanced by the presence of proton donors3). There is an increasing amount of information regarding chemical reactions with organic compounds 4. * Attach6 de Recherches au C.N.R.S. ** To whom correspondence should be addressed (50, av. F. D. Roosevelt, Brussels 1050, Belgium). *** Aspirant au F.N.R.S. * As independently shown by Kastening and Kazemifard is, triphenylphosphinoxide behaves in the same way.