A novel intermediate of the photosubstitution of [Ru(bpy) 2(dmbpy)] 2+ (dmbpy=3,3'-dimethyl-2,2'-bipyridine) with a monodentate dmbpy was identified and its structure in solution was determined. Quantum yields of the formation of the intermediate and of the net photosubstitution of the complex were measured in various solvents. Rate of the thermal rechelation of the monodentate dmbpy in the intermediate was also measured. The data indicated that the formation of the intermediate proceeded by dissociative mechanism. Refered to the dmbpy complex the reactions of[Ru(bpy) 3] 2+ and [Ru(phen) 3] 2+ were concluded to proceed by dissociative interchange mechanism, and [Ru(bpy) 2(L) 2] 2+ (L=pyridine, imidazole, NH 3, ethylenediamine/2, and picolylamine/2) by dissociative mechanism.