Spectra of the mixed-valence complexes [(bpy)(2)ClOs(L)Ru(NH3)(5)](4+) (L = 4,4’-bipyridine, pyrazine; bpy = 2,2’-bipyridine) are highly solvent dependent. They provide oxidation state specific spectral markers which show that oxidation states for L = 4,4’-bpy are Os-III-Ru-II in solvents of donor number (DN) < 14. In solvents of high donor number (DN > 15) they are Os-II-Ru-III. The isomers coexist at donor numbers 14-15 and the distribution between them tuned in mixtures of acetonitrile and propylene carbonate demonstrating that intramolecular electron transfer can be induced by varying the solvent. Both Os-III-Ru-II and Os-II-Ru-III display broad, solvent-dependent intervalence transfer (IT) bands in the near infrared (NIR). There are two isomers of [(bpy)(2)ClOs(pz)Ru(NH3)(5)](4+) as well. In one, which is dominant in solvents of DN < 22, the oxidation states are [(bpy)(2)ClOsIII(pz)Ru-II(NH3)(5)](4+) This is shown by the appearance of d pi --> d pi marker bands for Os-III (at 4200 and 5800 cm-l in CD3CN), nu(bpy) Os-III bands in the mid-IR, and the pattern of UV-visible bands.