The full geometry optimization of the A and B forms of bianthrone and their eight corresponding ions has been performed. From the resulting molecular structure parameters, some structure-bonding relationships are established. The calculated electron affinities, which take into account internal reorganizational energies, suggest that the bianthrasemiquinone radical anion B.- may be the first semiquinone that is stable toward oxidation by dioxygen. This is experimentally proven using electron paramagnetic resonance (EPR) spectroscopy. The B.- radical anion, prepared via the addition of NaOH to B in dimethyl sulfoxide (DMSO) solution, displays a strong well-resolved EPR spectrum. From the simulation of the spectrum and analysis of the line widths, the bimolecular rate constant for the spin-spin exchange between two B.- radicals is determined to be 3.19 x 10(9) L mol(-1) s(-1). When the solvent is saturated with O-2, evidence for the close proximity and the frequent encounters between the radical and O-2 is; apparent from the large broadening of the hydrogen hyperfine splittings. The magnitude of the line width broadening reveals that the Heisenberg spin-spin exchange between B.- and O-2 is approximately 10% less efficient than that of neutral nitroxide spin labels. This suggests that the DMSO solvation sphere surrounding the B.- radical anion protects it and slightly decreases its collision frequency with O-2. When the O-2 is expelled from solution, the EPR spectrum reverts to its normal well resolved form with no measurable loss of intensity. The unusual stability of B.- is attributed to the delocalization of its unpaired type electron over a major portion of this large anion.