Thermal denaturation studies were performed on a group of mononuclear mono- and diaqua polypyridyl complexes of Ru(II) covalently bound to calf thymus DNA. Adducts of monofunctional complexes show small, positive values of DELTAT(m) (0.8-3.5). The adducts of difunctional complexes exhibit much larger values (6-13), and the thermal denaturation is irreversible, which is consistent with formation of an interstrand diadduct. The dinuclear complex [(bpy)2Ru(OH2)]2O4+ was also studied and found to bind stereoselectively to calf thymus DNA (bpy = 2,2-bipyridine). Circular dichroism spectroscopy showed the filtrate obtained from ultrafiltration of calf thymus DNA and [(bpy)2Ru(OH2)]204+ to be enriched in one enantiomer. We also report here the synthesis and electronic properties of the model complex [(bpy)2(EtG)RuOH2]2+ (EtG = 9-ethylguanine). The complex is stable and possesses all of the characteristic electronic properties of the other polypyridylaquamthenium complexes. Surprisingly, the Ru(IV)O form is accessible via electrochemical oxidation and is an effective DNA cleavage agent.