Formation of adducts between Ru(TAP)3(2+) (TAP = 1,4,5,8-tetraazaphenanthrene) and DNA has been monitored by gel electrophoresis, UV-vis spectroscopy and dialysis methods. Adduct formation is found for both single- and double-stranded nucleic acids. The reaction with double-stranded DNA is found to be insensitive to solution pH or aeration. Spectroscopic changes similar to those for DNA are found with GMP in oxygen-free pH 5 solution. However, different reactions occur with GMP at higher pH or when the solution contains oxygen. Comparative experiments with double-stranded poly[d(G-C)] or poly[d(A-T)] indicate that the adduct with DNA involves binding to the guanosine moiety. It is proposed that the product is formed by the reaction of the reduced ruthenium complex and oxidised guanine species produced by photo-induced electron transfer.