Reactions of composition-selected cerium oxide cluster cations, CenOm+ (n = 2-6; m <= 2n), with O-2 have been studied under single collision conditions. The formation of CenOm+2+ is observed only for CenOm+ with the average oxidation number (AON) of similar to 3 for Ce atoms, Ce(III) clusters. In contrast, Ce(II) clusters with AON < 3 dominantly give the product ions, CenOm+1+, Cen-1Om+, and Cen-2Om-2+, after the oxidation of the parent clusters. Combined with collision-induced dissociation studies, it is concluded that O-2 is bound weakly to the Ce(III) cluster, whereas O-2 added to the Ce(II) cluster is incorporated as atoms rapidly and then the cluster decomposes. (C) 2013 Elsevier B.V. All rights reserved.