The interaction and photoreaction of a series of ruthenium(II) complexes containing 1,4,5,8,9,12-hexaazatriphenylene (hat) and 1,4,5,8-tetraazaphenanthrene (tap) ligands with nucleotides and DNA have been studied. The rate constant of quenching of the excited states of the complexes by guanosine-5’-monophosphate (GMP) is shown to depend on the reduction potentials of the metal complex excited state, suggesting that the quenching is due to electron transfer from the guanine. The more strongly oxidizing metal complex excited state species are also quenched by adenosine-5’-monophosphate (AMP). Electron transfer has been verified for Ru(hat)(3)(2+), Ru(tap)(3)(2+), and Ru(tap)(2)(hat)(2+) by laser flash photolysis, which indicates the formation of Ru(I) species and oxidized nucleotide intermediates with cage escape yields in the range 20-35%. Application of the Marcus theory yields a value of 1.16 V (vs NHE) for E(G.+/G) in GMP. The luminescence from Ru(hat)(3)(2+), Ru(tap)(3)(2+), Ru(tap)(2)L(2+), or Ru(hat)(2)L(2+) (L = 2,2’-bipyridine or 1,10-phenanthroline) is also quenched when the complexes are bound to DNA, and these oxidizing complexes are shown to be more efficient photosensitisers for single strand breaks in plasmid DNA. Covalently bound adducts are formed between the metal complexes and calf thymus DNA for those complexes whose excited states can oxidize guanine.