Near-UV or visible light irradiation of Ru(tap)(2)(bpy)(2+) (1) (tap = 1,4,5,8-tetraazaphenanthrene; bpy = 2,2'-bipyridyl) in the presence of duplex DNA induces the formation:of covalent adducts with guanine. The adduct has been isolated from the photomodified DNA as both its nucleotide and nucleobase derivatives by using a combination of enzymatic and acid hydrolytic procedures in conjunction with HPLC. Characterization by electrospray mass spectrometry and NMR spectroscopy shows that two isomeric covalent adducts are formed in which the exocyclic amino group of a guanine nucleobase is linked to the C2 or C7 position of one of the tap ligands. It is proposed that the products are generated from the reduced ruthenium complex and the guanine radical cation resulting from photoinduced electron transfer between 1 and guanine.