The energy transfer of a charge-transfer triplet excited state ((CT)-C-3) in [Ru(bpy)(3)](PF6)(2) (bpy = 2,2,＇-bipyridine) crystals doped with [Os(bpy)(3)](PF6)(2) was studied by a time-correlated single-photon counting method, where the excited Ru(II) moiety and the Os(II) moiety act as energy donor (D) and acceptor (A), respectively. (CT)-C-3 of Ru2+ in the [OsxRu1-x(bpy)(3)](PF6)(2) (x > 0.0099) exhibited multiexponential decay, which is ascribed to direct energy transfer to Os2+ at various rates depending on the D-A distances. Hopping of (CT)-C-3 to the closest Ru2+ (Ikeda et al., J. Phys. Chem. A, 2000) changed the D-A distances and made the decay faster at a later time. The complex emission decay was analyzed by a Monte Carlo simulation. The rates of energy transfer to Os2+ at 0.82 and 1.08 nm were determined to be 1.7 x 10(11) and 7 x 10(9) s(-1), respectively. The distance dependence of the rates of energy transfer through space and its mechanism are discussed.