Luminescence lifetimes (tau(m)) Of the sigma-bond-to-ligand charge-transfer (SBLCT) excited states of two diastereomers of fac-tris [(8-quinolyl)phenylmethylsilyl] iridium(III) differ by about a factor of 2 and are strongly solvent dependent. The tau(m) values of the more symmetric Delta RRR, Delta SSS diastereomer (A) are generally longer than those of the less symmetric Delta RRS, Delta SSR diastereomer (B); tau(m)’s of both diastereomers are substantially shortened relative to their values in aliphatic hydrocarbons by exciplex formation with a variety of weakly coordinating solvents including aromatic hydrocarbons, olefins, ethers, ketones, alcohols, and nitriles. Quenching constants (k(q)) due to exciplex formation are found to be much larger for B than they are for A in the a-donor solvents (cyclic ethers, ketones, alcohols, and nitriles); however, k(q) values of B are slightly smaller than those of A in pi-acceptor solvents (aromatic hydrocarbons, olefins). The results suggest that a-donor solvents form exciplexes by binding at the metal center, whereas pi-acceptor solvents bind at a quinolyl radical anion ligand site. A and B may prove useful as luminescent environmental probes which can distinguish between sigma-donor and pi-acceptor binding sites.