We report the synthesis, structure, and photophysical and electroluminescent (EL) properties of a series of heteroleptic bis(pyridylphenypiridium(III) complexes with various ancillary guanidinate ligands. The reaction of the bis (pyridylphenyl)iridium(III) chloride [(ppy)(2)Ir (mu-Cl)](2) with the lithium salt of various guanidine ligands Li{((NPr)-Pr-i)(2)C-((NRR2)-R-1)} at 80 degrees C gave in 60-80% yield the corresponding heteroleptic bis(pyridylphenyl)/guanidinate iridium(III) complexes having a general formula of [(ppy)(2)Ir{((NPr)-Pr-i)C-2-((NRR2)-R-1)}], where (NRR2)-R-1 = NPh2 (1), N((C6H4Bu)-Bu-t-4)(2) (2), carbazolyl (3), 3,6-bis(tert-butyl)carbazoly1 (4), N(C6H4)(2)S (5), N(C6H4)(2)O (6), indolyl (7), NEt2 (8), (NPr2)-Pr-i (9), (NBu2)-Bu-i (10), and N(SiMe3)(2) (11). These heteroleptic cyclometalated (C boolean AND N) iridium(III) complexes showed intense absorption bands in the UV region assignable to pi-pi* transitions and weaker metal-to-ligand charge-transfer transitions extending to the visible region. These complexes also showed intense emissions at room temperature. Their photoluminescence spectra were influenced to some extent by the ancillary guanidinate ligands, giving lambda(max) values in the range of 528-560 nm with quantum yields (Phi) of 0.16-0.37 and lifetimes of 0.61-1.43 mu s. Organic fight-emitting diodes were fabricated by the use of these complexes as dopants in various concentrations (5-100%) in a N,N'-dicarbazolylbiphenyl host. High current efficiency (eta(c); up to 137.4 cd/A) and power efficiency (eta(p); up to 45.7 lm/W) were observed under appropriate conditions. Their high EL efficiency may result from efficient trapping and radiative relaxation of the excitons formed in the EL process. Because of the steric hindrance of the guanidinate ligands, no significant intermolecular interaction was observed in these complexes, thus leading to the reduction of self-quenching and triplet-triplet annihilation at high currents. The EL emission color could be changed in the range of green to yellow by choosing appropriate guanidinate ligands.