Cationic complexes of the type [M(CO)S(PPh(3))(2)](+) (M = Ir, Rh; S = CH3CN) react with singlet oxygen to form the corresponding peroxo complexes [M(CO)S(PPh(3))(2)(O-2)](+). The solvent molecule remains coordinated to the metal in the oxygen adducts. The novel cationic iridium-peroxo complex is stable at room temperature, while the rhodium-peroxo complex is only stable below 0 degrees C. Rate constants for physical and chemical interaction of the complexes with singlet oxygen are somewhat smaller than those for related neutral complexes. Upon addition of alkenes (tetramethylethylene or 1-octene) to the peroxo complexes, neither oxidation of the olefins nor substitution of the acetonitrile ligand was observed. 1-Octene was isomerized to give mostly 2- and 3-octene by the cationic rhodium(I) complex. A cationic iridium complex which already possesses a coordinated diene ligand ([Ir(COD)(PPh(3))(2)](+)) did not react with or quench singlet oxygen.