In crystals of double-complex salts [M(bpy)(3)](2)[Cr(CN)(6)]Cl . 8H(2)O (M2+ = Ru2+, Os2+; bpy = 2,2'-bipyridine), luminescence from (CT)-C-3 state of [M(bpy)(3)](2+) is partially quenched by [Cr(CN)(6)](3-) at 77 K and room temperature (RT). This quenching is attributed to intermolecular excitation energy transfer from the (CT)-C-3 state of [M(bpy)(3)](2+) to the E-2(g) state of [Cr(CN)(6)](3-). Crystal structure and crystal parameters of [Os(bpy)(3)](2)[Cr(CN)(6)]Cl . 8H(2)O: monoclinic, C2, a = 22.384(4) Angstrom, b = 13.827(4) Angstrom, c = 22.186(3) Angstrom, beta = 90.70(2)degrees, V = 6866(2) Angstrom(3), Z = 4, R = 0.0789, R-w = 0.1932: are almost the same as those of [Ru(bpy)(3)](2)[Cr(CN)(6)]Cl . 8H(2)O: monoclinic, C2, a = 22.414(2) Angstrom, b = 13.7686(15) Angstrom, c = 22.207(2) Angstrom, beta = 90.713(8)degrees, V = 6852.9(12) Angstrom(3), Z = 4, R = 0.0554; R-w = 0.1679. Moreover, these double complex salts have the same distance and relative orientation between donor and acceptor. The rate of intermolecular energy transfer from [M(bpy)(3)](2+) to [Cr(CN)(6)](3-) was evaluated by the decay time of luminescence from (CT)-C-3 state of [M(bpy)(3)](2+) in single- and double-complex salts. The rate of energy transfer in [Os(bpy)(3)](2)[Cr(CN)(6)]Cl . 8H(2)O (4.9 x 10(7) s(-1)) is about eight times larger than that in [Ru(bpy)(3)](2)[Cr(CN)(6)]Cl . 8H(2)O (6.0 x 10(6) s(-1)) at 77 K. The difference of energy transfer rate is brought about by only the spectral overlap between the normalized luminescence spectrum from the (CT)-C-3 state of donor ([M(bpy)(3)](2+)) and the normalized excitation spectrum of the E-2(g) state of acceptor ([Cr(CN)(6)](3-)) in the salts. Decay rates of the (CT)-C-3 state in [M(bpy)(3)](2)[Cr(CN)(6)]Cl . 8H(2)O were measured as a function of temperature. A large enhancement of a decay rate from the (CT)-C-3 state was obtained for [Ru(bpy)(3)](2)[Cr(CN)(6)]Cl . 8H(2)O as the temperature was increased. This result implies that an additional path from the (CT)-C-3 state of [Ru(bpy)(3)](2+) to the T-2(2g) state of [Cr(CN)(6)](3-) would be opened for energy transfer with a rise in temperature in [Ru(bpy)(3)](2)[Cr(CN)(6)]Cl . 8H(2)O.