The mechanism for triplet energy transfer from the green-emitting fac-tris[2-(4'-tert-butylphenyl)pyridinato]iridium (Ir(tBu-ppy)3) complex to the red-emitting bis[2-(2'-benzothienyl)pyridinato-N,C3')(acetylacetonato)iridium (Ir(btp)2(acac)) phosphor has been investigated using steady-state and time-resolved photoluminescence spectroscopy. [2,2';5,'2' ']Terthiophene (3T) was also used as triplet energy acceptor to differentiate between the two common mechanisms for energy transfer, i.e., the direct exchange of electrons (Dexter transfer) or the coupling of transition dipoles (Förster transfer). Unlike Ir(btp)2(acac), 3T can only be active in Dexter energy transfer because it has a negligible ground state absorption to the 3(-*) state. The experiments demonstrate that in semidilute solution, the 3MLCT state of Ir(tBu-ppy)3 can transfer its triplet energy to the lower-lying 3(-*) states of both Ir(btp)2(acac) and 3T. For both acceptors, this transfer occurs via a diffusion-controlled reaction with a common rate constant (ken = 3.8 × 109 L mol-1 s-1). In a solid-state polymer matrix, the two acceptors, however, show entirely different behavior. The 3MLCT phosphorescence of Ir(tBu-ppy)3 is strongly quenched by Ir(btp)2(acac) but not by 3T. This reveals that under conditions where molecular diffusion is inhibited, triplet energy transfer only occurs via the Förster mechanism, provided that the transition dipole moments involved on energy donor and acceptor are not negligible. With the use of the Förster radius for triplet energy transfer from Ir(tBu-ppy)3 to Ir(btp)2(acac) of R0 = 3.02 nm, the experimentally observed quenching is found to agree quantitatively with a model for Förster energy transfer that assumes a random distribution of acceptors in a rigid matrix.