Solutions of Rh(PR3)(2)(CO)Cl (R = Me, Ph) are found to catalyze the rapid transfer of oxygen from amine oxides or organoselenium oxides to carbon monoxide; however, the rhodium complexes undergo no reaction with the oxides in the absence of added CO. Kinetic studies indicate that the catalytically active species is the CO-substituted complex Rh(PR3)(CO)(2)Cl, although it is not present in any observable concentration under the conditions of the reaction. Ir(PPh3)(2)(CO)(2)Cl also acts as an efficient catalyst precursor for the same oxygen transfer reactions, although like the rhodium complex it undergoes little or no direct reaction with the oxides. The catalytically active species is again found to be the product of substitution of a ligand (in this case, chloride) by CO : [Ir(PPh3)(2)(CO)(3)](+) in either ion-paired or unpaired states. Among substrates with weak E-O bonds (E = N, Se), reactivity correlates with substrate basicity in accord with a transition state having the character of a nucleophilic attack (at carbonyl carbon). Oxides with much stronger E-O bonds, even the highly basic triphenylarsine oxide, are much less reactive; the transition state in this case apparently involves significant E-O bond breaking and is presumably not well modeled as a simple nucleophilic attack. Pt(Ph3As)(CO)Cl-2 was found to act asa good catalyst precursor for deoxygenation of arsine oxide, but this system is apparently very complex and the nature of the-catalytically active species has not been elucidated.