The kinetic energy dependence of the reactions of Fe-n(+) (n = 2-18) with O-2 are studied in a guided ion beam mass spectrometer. A variety of FemO2+, FemO+, and Fe-m(+), where m less than or equal to n, product ions art 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 products is analyzed in several different ways to determine thermochemistry for bath the first and second oxygen atom binding to iron cluster ions. The trends in this thermochemistry are discussed and compared to bulk phase values.