The density functional theory calculation has been used to investigate the formation and transformation of superoxide species at La-O clusters under both the singlet and triplet states. The calculations predict that the superoxide species could be formed directly from gas oxygen close to neutral or negatively charged clusters, but at the positively charged cluster could be formed indirectly via ozone species. The formation of ozone species may result in an oxygen exchange between gas oxygen and La-O clusters. The most stable superoxide species would be formed at the negatively charged clusters. (c) 2006 Elsevier B.V. All rights reserved.