A series of metal complexes that combine the structure of phenanthroline and salen have been synthesized and characterized by electron paramagnetic resonance spectroscopy. The effects of the 2,9-bis(2-hydroxyphenyl)-1,10-phenanthroline compounds complexed with CuII, NiII, CoII or MnIII on the temperature-dependent helix-to-coil transition of DNA have been measured. The interaction with DNA is metal-dependent and the highest stabilization is observed with the Co complex. The DNA cleaving activities have been studied with plasmid DNA and/or with a 32P-labelled duplex oligonucleotide depending on the redox properties of the complexes. The Cu complex is inactive whereas the Co chelate efficiently cleaves DNA in the presence of a reducing agent. Cleavage of DNA by the Mn complex can occur either in the presence of a reducing agent via the production of oxygen radicals (which are detected by EPR spectroscopy) or in the presence of an oxidant such as KHSO5. In both cases, the cleavage of nucleic acids is very efficient whereas no cleavage is observed with the Ni complex. The complexes of bis(hydroxyphenyl)phenanthroline with Mn and Co complement the tool-box of reagents available for cleavage of DNA.