The interaction of molecular oxygen with polycrystalline Bi₂O₃ was investigated by infrared (IR) and thermogravimetric (TGA) analyses, X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). The results indicate the formation of “end-on” and “bridging” superoxide species, which are trapped in the β-Bi₂O₃ vacant oxygen sites, Bi(IV) and Bi(V) centres also being produced. O₂ ⁻ interacts with Bi(IV) centres giving two superoxide adducts with temperature-dependent relative amounts, Bi(IV)–O₂ ⁻ and Bi(IV)–O₂ ⁻–Bi(III), both having a triplet spin state.