Interligand electron transfer kinetics have been measured in Os-II(bipyridine)(3) following photoexcitation to the metal-to-ligand charge-transfer state. The measurements are made in room temperature solutions of acetonitrile, ethylene glycol, and glycerol, using time-resolved absorption polarization spectroscopy. The experimental results are strongly dependent on excitation wavelength and are in agreement with the results of reaction/diffusion model calculations. In the calculations, motion along the solvent polarization coordinate is treated as diffusion, and reaction is treated in the nonadiabatic (acetonitrile) or adiabatic (ethylene glycol) limits. The adjustable parameters of the model are the interligand electronic coupling and the solvent polarization barrier height, which are taken to be 15 and 500 cm(-1), respectively. The relaxation processes giving rise to the observed kinetic components are discussed.