The kinetic energy dependence of the reactions of Cr-n(+) (n = 2-18) with O-2 are studied in a guided ion beam mass spectrometer. A variety of CrmO2+, CrmO+, and Cr-m(+) product ions, where m less than or equal to n, 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 dependence of the product cross sections is analysed in several different ways to determine thermochemistry for both the first and second oxygen atom binding to chromium 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.