The solvent effect on the radiationless transition rates from the excited triplet MLCT (metal-to-ligand charge-transfer) states of tris(bipyridine)osmium(II) (Os(bpy)(3)(2+); bpy = 2,2'-bipyridine) and tris(phenanthroline)osmium(II) (Os(phen)(3)(2+); phen = l,10-phenanthroline) was investigated in water-methanol mixed solvent and its deuterated analogues at various temperatures. Similar measurements were carried out in H2O-D2O and H2O-dioxane mixed solvents. A significant isotope effect of the solvent on the rate constant of the radiationless relaxation to the ground state was observed in water, while only a small, one was recognized in methanol. The radiationless transition rate in the H2O-D2O mixture depends linearly on the mole fraction of D2O, while that in the H2O-methanol mixture shows no linear dependence on the mole fraction of methanol. Those solvent effects of the radiationless transition are described as the energy transfer from the metal complex to the water molecule included in the Os(II) complex. It was suggested that the observed radiationless transition rates were strongly affected by the microscopic environment around the Os(II) complex from the results in the mixed solvents.