The reaction between molecular oxygen and rhodium hydrides L(OH)RhH+ (L = (NH3)(4), trans-L-1, and cis-L-1, where L-1 = cyclam) in basic aqueous solutions rapidly produces the corresponding hydroperoxo complexes. Over the pH range 8 < pH < 12, the kinetics exhibit a first order dependence on [OH-]. The dependence on [O-2] is less than first order and approaches saturation at the highest concentrations used. These data suggest an attack by OH- at the hydride with k = (1.45 +/-0.25) x 10(3) M-1 s(-1) for trans-L-1(OH) RhH+ at 25 degrees C, resulting in heterolytic cleavage of the Rh - H bond and formation of a reactive Rh(I) intermediate, A competition between O-2 and H2O for Rh(I) is the source of the observed dependence on O-2. In support of this mechanism, there is a significant kinetic isotope effect for the initial step, L-1(OH(D))RhH(D)(+) + OH(D)(-) reversible arrow(k1)(k-1) L-1(OH(D))Rh-1 + H(D)(2)O, k(1H)/k(1D) = 1.7, and k(-1H)/k(-1D) = 3.0. The activation parameters for k(1) for trans L1(OH)RhH+ are Delta H double dagger = 64.6 +/- 1.3 KJ mol(-1) and Delta S-t = 40 +/- 4 J mol(-1) K-1.