Energy transfer in a single crystal of [Rh(phpy)(2)bpy]PF6, substitutionally doped with 0.9% [Ir(thpy)(2)bpy](+) (phpy(-) = 2-phenylpyridine, bpy = 2,2’-bipyridine, and thpy(-) = 2,2’-thienylpyridine), is studied from picosecond transient phosphorescence measurements in the temperature range from 130 up to 290 K. The cation complexes of host and guest compounds both exhibit a lowest excited (3) pi pi* state, localized on one phpy(-) or thpy(-) ligand, respectively. The emission spectrum of the undoped crystal is characterized by luminescence from an A trap, 98 cm(-1) below the origin of the host absorption. In the mixed crystal, the emission spectrum is dominated by luminescence from the deep Ir3+ trap (3200 cm(-1) below the host band). After picosecond pulsed laser excitation, a decay of the phosphorescence intensity of the A trap within about 100 ns is observed; the decay of the A-trap emission is accompanied by a rise in the intensity of the Ir3+-trap phosphorescence on the same time scale. The A-trap luminescence decay is attributed to intermolecular energy transfer, and its time behavior can be simulated using a 3D-exchange interaction model. In addition, it is found that, upon increasing the temperature, the characteristic time for hopping between nearest donor species is increased. It is argued that, at higher temperatures, the excitation hopping is among spatially extended donor states, characteristic of eigenstates of donor clusters. Due to the longer lifetime of these cluster eigenstates intercluster energy transfer is effectively slowed down.