The kinetic energy dependences of the reactions of Ni-n(+) (n=2-18) with O-2 are studied in a guided ion beam tandem mass spectrometer. A variety of NimO2+, NimO+, and Ni-m(+) product ions, where mless than or equal ton, are observed, with the dioxide cluster ions dominating the products for all larger reactant cluster ions. Reaction efficiencies are near unity for all but the smallest clusters. The energy dependences of the product cross sections are analyzed in several different ways to determine thermochemistry for both the first and second oxygen atom binding to nickel cluster ions. These values show little dependence on cluster size for clusters larger than three atoms. The trends in this thermochemistry are discussed and compared to bulk phase oxidation values. (C) 2003 American Institute of Physics.