The photoinduced ligand exchange reaction of Ru(II) complexes in aqueous solution was studied using density functional theory (DFT). The optimized structures of the lowest triplet state of cis-[Ru(bpy)(2)(CH3CN)(2)](2+) (bpy = bipyridine), cis-[Ru(bpy)(2)(NH3)(2)](2+), and their monoaqua complexes were analyzed. The metal-centered ((MC)-M-3) structure was lower than the metal-to-ligand charge transfer ((MLCT)-M-3) structure for cis-[Ru(bpy)(2)(CH3CN)(2)](2+), whereas the (MLCT)-M-3 structure was lower than the (MC)-M-3 structure for cis[Ru(bpy)(2)(NH3)(2)](2+). Such a difference would correlate with the higher quantum yield of the former complex. For the monoaqua complexes, the most stable local minimum structure was the (MC)-M-3 structure, in which the Ru-O-H2O and Ru-N-bpy (trans to the oxygen) bonds were elongated. Therefore, the dissociation of the H2O ligand would be preferred to that of the CH3CN (or NH3) ligand from the monoaqua intermediate, which might result in the reformation of the monoaqua intermediate, and thus, the formation of the bis-aqua product would take a longer time than that of the monoaqua intermediate.