The reactions of nickel cluster cations, Ni-n(+) (n = 2-29), with O-2 and their sequential reactions were studied using FT-ICR mass spectrometry. Reaction products were observed after 10(-1)-10(3) collisions with O-2. The reaction probabilities of Ni-n(+) (n = 3-15) with O-2 were almost unity while that of Ni-2(+) was around 0.5. After 10(2)-10(3) collisions with O-2, the fully oxidized nickel clusters with the bulk nickel oxide composition (Ni/O = 1) were produced. However, the dominant products were two kinds of nickel suboxide clusters, Ni13O8+ and Ni16O10+, which were inert in Q, under the present conditions. (c) 2005 Elsevier B.V. All rights reserved.