A heterogeneity of surface sites for superoxide (O-2(-)) stabilization on thermally reduced TiO2 (P25) have been identified by cw EPR. Two main groups of adsorption sites exist; site I due to O-2(-) adsorbed at oxygen vacancies, [Vac center dot center dot center dot O-2(-)], and sites II-III due to O-2(-) adsorbed at five co-ordinate Ti4+ centres. Electron transfer from the precursor Ti3+ centres to O-2 (forming O-2(-)) can be prevented at each site through selective site blocking with probe molecules (CO2 and Ar). CO2 preferentially adsorbs at the five co-ordinate Ti-non-vacancy(3+) non-vacancy centres, while Ar preferentially adsorbs at the Ti3+ centres associated with the oxygen vacancies (Ti-vacancy(3+)). (C) 2009 Elsevier B. V. All rights reserved.