The synthesis and photophysical study of a family of cyclometalated iridium(III) complexes are reported. The iridium complexes have two cyclometalated (C ∧ N) ligands and a single monoanionic, bidentate ancillary ligand (LX), i.e., C ∧ N 2Ir(LX). The C ∧ N ligands can be any of a wide variety of organometallic ligands. The LX ligands used for this study were all β-diketonates, with the major emphasis placed on acetylacetonate (acac) complexes. The majority of the C ∧ N 2Ir(acac) complexes phosphoresce with high quantum efficiencies (solution quantum yields, 0.1−0.6), and microsecond lifetimes (e.g., 1−14 μs). The strongly allowed phosphorescence in these complexes is the result of significant spin−orbit coupling of the Ir center. The lowest energy (emissive) excited state in these C ∧ N 2Ir(acac) complexes is a mixture of 3MLCT and 3(π−π*) states. By choosing the appropriate C ∧ N ligand, C ∧ N 2Ir(acac) complexes can be prepared which emit in any color from green to red. Simple, systematic changes in the C ∧ N ligands, which lead to bathochromic shifts of the free ligands, lead to similar bathochromic shifts in the Ir complexes of the same ligands, consistent with “C ∧ N 2Ir”-centered emission. Three of the C ∧ N 2Ir(acac) complexes were used as dopants for organic light emitting diodes (OLEDs). The three Ir complexes, i.e., bis(2-phenylpyridinato-N,C2‘)iridium(acetylacetonate) ppy 2Ir(acac), bis(2-phenyl benzothiozolato-N,C2‘)iridium(acetylacetonate) bt 2Ir(acac), and bis(2-(2‘-benzothienyl)pyridinato-N,C3‘)iridium(acetylacetonate) btp 2Ir(acac), were doped into the emissive region of multilayer, vapor-deposited OLEDs. The ppy 2Ir(acac)-, bt 2Ir(acac)-, and btp 2Ir(acac)-based OLEDs give green, yellow, and red electroluminescence, respectively, with very similar current−voltage characteristics. The OLEDs give high external quantum efficiencies, ranging from 6 to 12.3%, with the ppy 2Ir(acac) giving the highest efficiency (12.3%, 38 lm/W, >50 Cd/A). The btp 2Ir(acac)-based device gives saturated red emission with a quantum efficiency of 6.5% and a luminance efficiency of 2.2 lm/W. These C ∧ N 2Ir(acac)-doped OLEDs show some of the highest efficiencies reported for organic light emitting diodes. The high efficiencies result from efficient trapping and radiative relaxation of the singlet and triplet excitons formed in the electroluminescent process.