Cross sections for the reactions of Fen+ and Nb n+ (n = 1-3) with O2 are measured as a function of kinetic energy over a range of 0 to > 10 eV. In all systems, analysis yields insight into the kinetics and thermochemistry of the oxidation processes. Nbn+ reaction with O2 exothermically near the Langevin-Gioumousis-Stevenson close-collision limit, driven by formation of strong NbO+ and NbO bonds. Fen+ are less reactive, although oxidation becomes progressively more facile as the size of the reactant increases from Fe+ to Fe3+. In contrast to the Nbn+ systems, Fen+ (n = 2,3) react at elevated energies by simple cluster fragmentation processes. Quantitative limits are established for ionic and neutral cluster oxide bond dissociation energies. Cross sections for formation of MnO + from reaction of Mn+ (Fe3+, Nb2+, and Nb3+ ) are observed to have both an exothermic and an endothermic feature. Since there is only one chemical pathway to form this product, it is suggested that there are activation barriers to formation of favorable reaction intermediates. A similar suggestion is required to explain product branching ratios involving metal dioxides which run counter to thermodynamic predictions. © 1989 American Institute of Physics.