Dicationic metal superoxo complexes, L(H2O)MOO2+ (M = Rh, Cr; L = (NH3)(4), (H2O)(5), [14]aneN(4)) react with rhodium hydrides L'(H2O)RhH2+ (L' = (NH3)(4), [14]aneN(4), and meso-Me-6-[14]aneN(4)) to yield L(H2O)MOOH2+ and L'(H2O)(2)Rh2+. The rhodium(II) product is rapidly converted to L'(H2O)RhOO2+ in a reaction with molecular oxygen (k similar to 10(8) M-1 s(-1)), or to a binuclear superoxo product in the absence of O-2. When the central metal atom and nonparticipating ligands are identical in the two reactants (M = Rh, L = L'), the O-2 insertion into Rh-H bond becomes catalytic, L'RhH2+ + O-2 --> L'RhOOH2+ (L'RhOO2+ = catalyst). The hydrogen transfer step exhibits a large kinetic isotope effect (k(H)/k(D) = 7.6 for the reaction of CraqOO2+ with ([14]aneN(4))(H2O)RhH2+, Delta H-H(double dagger) = 20.3 +/- 1.8 kJ mol(-1), Delta S-H(double dagger) = -136 +/- 6 J mol(-1) K-1, and Delta H-D(double dagger) = 24.7 +/- 4.3 kJ mol(-1), Delta S-D(double dagger) = -138 +/- 15 J mol(-1) K-1), but the isotopic composition of the solvent has no effect on rates, k(H2O)/k(D2O) = 1. Steric effects play a major role in the kinetics. From the available thermochemical data, the energy of the CraqOO-H2+ bond was calculated to be similar to 330 kJ/mol.