An intermediate of the photosubstitution of [Ru(bpy)2(dmbpy)]2+ (dmbpy = 3,3’-dimethyl-2,2’-bipyridine) with a monodentate dmbpy ligand has been identified by an absorption spectrum of the intermediate separated by HPLC and a H-1 NMR spectrum of a photoirradiated solution of the metal complex. Thermal back-reaction of the intermediate to the original complex was confirmed by H-1 NMR and HPLC methods. This is the first example of the identification of a reaction intermediate with a monodentate diimine ligand for a tris(diimine) type complex.