Pyridinone or pyrimidinone nucleoside bases containing fused aromatic polycyclic rings are provided. These polycyclic nucleosides are incorporated into oligonucleotides and hybridized to complementary nucleic acid. Fluorescence spectroscopy and thermal denaturation profiles provided evidence that the polycyclic base is intercalated into the resulting duplex. The fused polycyclic ring systems optionally are substituted with reactive species which inactivate complementary nucleic acids. The oligonucleotides of this invention are useful as improved probes, diagnostic reagents, or for cleaving or derivatizing predetermined domains within nucleic acids.