Density functional calculations using the B3LYP, BP86, and FT97 functionals with an extended basis set are employed to investigate the mechanisms of H-2 oxidation by FeO+. Three mechanisms are considered, addition-elimination, "rebound", and oxene-insertion. The oxene-insertion is characterized by high barriers and only second-order saddle points. The addition-elimination and "rebound" mechanisms are competitive and both exhibit two-state-reactivity (TSR) with a crossing between sextet and quartet states. TSR provides a low-energy path for bond activation and is predicted to be the dominant pathway at room temperature. Both TSR mechanisms are concerted : the addition-elimination mechanism involves 2+2 addition in the bond activation step, while the rebound mechanism is effectively concerted involving the H-abstraction followed by a barrierless "rebound" of the H-radical. At elevated temperature (above a threshold of 3/2 RT = 0.5 eV), the stepwise "rebound" mechanism starts to dominate and produces FeOH+ + H-. via a single-state-reactivity (SSR) on the sextet surface. Kinetic isotope effect calculations have been performed, and their comparison with the experimental data(10) seems to be characteristic of TSR. Thus, the measured isotope effects probe the extent of H...H (D...D) cleavage in a mechanism whereby bond activation and spin-inversion occur in a concerted manner. Some predictions have been made regarding the factors that affect the mechanistic competition.