In solution, singlet oxygen is usually prepared by using various sensitizers (a dye or pigment, a quinone or an aromatic compound, examples are Methylene Blue, Rose Bengal, C60). The excited sensitizer, usually in the triplet state, reacts with triplet oxygen to give singlet oxygen. Some organometallics are also photosensitizers for singlet dioxygen, but overall, there has been relatively little work in this area. We are examining the photophysical properties of the bis(cyclometallated) Ir(III) and Pt(II) complexes prepared by the group of Prof. Thompson at the University of Southern California. We have found that these complexes are excellent sensitizers for the production of singlet oxygen. The bis-cyclometallated Ir(III) complexes (BT)2Ir(acac), (BSN)2Ir(acac), and (PQ)2Ir(acac); BT = 2-phenylbenzothiazole, BSN = 2-(1-naphthyl)benzothiazole, PQ = 2-phenylquinoline, and acac = acetylacetonate) studied are depicted below. Complexes with acetylacetonate ancillary ligands give singlet oxygen quantum yields near unity (FD = (0.7 - 1.0) ± 0.1)., whether exciting the ligand-based state or the lowest energy excited state (MLCT + 3LC). Similar complexes were prepared with amino acids (glycine) or pyridine tethered to the Ir(III) center (i.e. BSN-G, and (BT-py; G =glycine, and py = pyridine).