Oxygen absorbed on TiO₂ when subjected to u.v. irradiation gives rise to three different paramagnetic species. The first species, A, has been identified as O^–₂(g₁= 2.021, g₂= 2.009 and g₃= 2.001 and C₁≈C₂≈ 0 Oe, C₃= 77 Oe for ¹⁷O hyperfine spliting), the second species, C, as O^–₃(g₁= 2.014, g₂= 2.009 and g₃= 2.003, no detected hyperfine lines due to ¹⁷O) and the third, B, as O^3–₃(g_∥= 2.008 and g_⊥= 2.001, no detectable hyperfine lines due to ¹⁷O). The latter species reacts readily with CO at 77 K and gives rise to species D, identified as O_I–O_II–CO^–_III according to the reaction: O^3–₃+ CO → O^2–_latt.+ O—O—CO^– where the two directly bonded oxygen atoms originate from the absorbed molecular oxygen. The e.p.r. parameters of species D are : g₁= 2.0465, g₂= 2.006, g₃= 2.001, C₁≃ 6 Oe ± 0.5C₂≈C₃≈ 4 Oe ± 0.5 for ¹³C hyperfine interaction, C^I₁≈C^I₂≈ 0 and C^I₃= 104 Oe, C^II₁C≈^II₂ 0 and C^II₃≃ 42.5 Oe and C^III₁≈C^III₂≈C^III₃≈ 0 for ¹⁷O hyperfine splitting constants of the three oxygen nuclei. It is mainly the characterization of species, D, which enables us to postulate the nature of species, B.