We report the singlet oxygen sensitization properties of a series of bis-cyclometalated Ir(III) complexes (i.e., (bt)2Ir(acac), (bsn)2Ir(acac), and (pq)2Ir(acac); bt = 2-phenylbenzothiazole, bsn = 2-(1-naphthyl)benzothiazole, pq = 2-phenylquinoline, and acac = acetylacetonate). Complexes with acetylacetonate ancillary ligands give singlet oxygen quantum yields near unity (PhiDelta = (0.7-1.0) +/- 0.1), whether exciting the ligand-based state or the lowest energy excited state (MLCT + 3LC). The singlet oxygen quenching rates for these beta-diketonate complexes were found to be small [(5 +/- 2) x 105 to (6 +/- 0.2) x 106 M-1 s-1], roughly 3 orders of magnitude slower than the corresponding phosphorescence quenching rate. Similar complexes were prepared with glycine or pyridine tethered to the Ir(III) center (i.e., (bsn)2Ir(gly) and (bt)2Ir(py)Cl; gly = glycine and py = pyridine). The glycine and pyridine derivatives give high singlet oxygen yields (PhiDelta = (0.7-1.0) +/- 0.1).