A series of terdentate cyclometallated Pt-II complexes with remarkable luminescence properties are used as new phosphorescence-emitting dopants in a blended host matrix as the emitting layer, resulting in very high electroluminescence efficiencies. Because of the high phosphorescence quantum yields of these Pt complexes and the efficient energy transfer from both singlet and triplet excited states of the host to the emitting guest, external electroluminescence quantum efficiencies as high as 4-16 % photons per carrier and luminous efficiencies of 15-40 cd A(-1) are achieved. Moreover, these high efficiency values were maintained over a four-decade current intensity span with no significant roll-off. Tuning of the electroluminescence spectra from the yellow to the green-bluish region of the chromaticity diagram is obtained simply by changing the substituents at the central 5-position of the cyclometallating ligand.