Oxidation processes of chromium cluster cations, Cr-2(+) and Cr-3(+), are investigated by kinetics measurements under multiple-collision conditions by using a temperature-controlled ion trap. Both of the reactants commonly generate Cr+ and CrO+ as final products. However, the major reaction pathway to CrO+ presents a sharp contrast; Cr-2(+) produces it directly by a single O-2 collision, whereas Cr-3(+) does via an intermediate Cr2O+. Comparison of the kinetics of the intermediate with that of cold Cr2O+ generated in the cluster source reveals that the intermediate Cr2O+ is thermalized promptly with the buffer gas filling the trap. (C) 2012 Elsevier B.V. All rights reserved.