The nature of oxygen adsorbed on slightly reduced TiO2 has been investigated using oxygen enriched with 17O2. On anatase, the oxygen adsorbs as O–2 with a hyperfine splitting Axx about g3 of 77 G; no structure could be resolved for Ayy and Azz. On rutile, oxygen adsorbs as O–2 on two sites of different thermal stability; (I) with g1= 2.030, g2= 2.008, g3= 2.004 and Axx= 76 G, and (II) with g1= 2.020, g2= 2.009, g3= 2.003 and Axx= 72 G. The evidence indicates that O–2 exists in a largely ionic form on TiO2 and other oxides. A symmetrical line at g= 2.003 which appears on some anatase samples after oxygen adsorption is attributed to the localization of a conduction electron by the adsorbed oxygen.