The Fe-57 Mossbauer spectrum of [Ru-IV(NH3)(4)(NH2)(Fe-II(CN)(6)H)] consists of a quadrupole doublet, showing that, in being formed in the reaction of [Ru(NH3)(5)H2O](3+) With Fe(CN)(6)(3-), the bridging ligand has undergone a rotation of 180 degrees, consistent with there being an induction period in the reaction profile for its appearance. In harmony with the foregoing, [Os-IV(NH3)(4)(NH2)(Fe-II(CN)(6)H)], the product of the analogous reaction, shows a single absorption peak, its appearance following a first-order reaction. Though an induction period is observed in the formation of [Os-III(NH3)(5)(Fe-II(CN)(6))](-), the Mossbauer spectrum of the potassium salt shows a single absorption peak. Our present more detailed study of the kinetics of substitution by Fe(CN)(6)(4-) and Co(CN)(6)(3-) in [Os(NH3)(5)H2O](3+) in unacidified solution shows that a catalyst for the reaction of the former is generated by the residual O-2 in a controlled-atmosphere box. The kinetic behavior of the reaction system with catalyst deliberately generated is described in this paper. Catalysis depends on labilization to substitution attending the oxidation of an Os(III) species to the Os(IV) state.