We have successfully applied electrospray ionization mass spectrometry (ESI-MS) and H-1 NMR analyses to study ligand substitution reactions of mu-oxo ruthenium bipyridine dimers cis,cis-[(bpy)(2)(L)RuORu(L')(bpy)(2)](n+) (bpy = 2,2'-bipyridine; L and L' = NH3, H2O, and HO-) with solvent molecules, that is, acetonitrile, methanol, and acetone. The results clearly show that the ammine ligand is very stable and was not substituted by any solvents, while the aqua ligand was rapidly substituted by all the solvents. In acetonitrile and acetone solutions, the substitution reaction of the aqua ligand(s) competed with a deprotonation reaction from the ligand. The hydroxyl ligand was not substituted by acetonitrile or acetone, but it exchanged slowly with CH3O- in methanol. The substitution reaction of the aqua ligands in [(bpy)(2)(H2O)(RuORuIII)-O-III(H2O)(bpy)(2)](4+) was more rapid than that of the hydroxyl ligand in [(bpy)(2)(H2O)(RuORuIV)-O-III(OH)(bpy)(2)](4+). In methanol, slow reduction of Ru-III to Ru-II was observed in all the mu-oxo dimers, and the Ru-O-Ru bridge was then cleaved to give mononuclear Ru(II) complexes.