Photosensitized oxidation of a series of iridium(I) complexes of the type trans-Ir(CO)X(PPh(3))(2) (X = halogen) leads to the same iridium(III) dioxygen complexes as the reaction with triplet oxygen. The reaction with singlet oxygen is many orders of magnitude faster than the triplet oxygen reactions. In contrast to those for the reaction with triplet oxygen, the rate constants for the O-1(2) reaction and physical deactivation do not vary significantly with different ligands, except for extremely electron-poor complexes, where there is no interaction between the complex and singlet oxygen. The analogous rhodium(I) complexes show very similar reactivity, The resulting previously unknown rhodium(III) dioxygen complexes are unstable at room temperature. Related square-planar platinum(II) complexes do not show any interaction with singlet oxygen, except for trans-PtHCl(PEt(3))(2), which gives some physical deactivation of singlet oxygen; however, with this compound, no reaction product could be detected even at low temperature. The results suggest that many metal complexes may react with singlet oxygen to form novel metal-dioxygen complexes.