Two complexes of [Co(en)2phen]3+ (1) and [Co(en)2bpy]3+ (2) have been synthesized and characterized by UV/VIS, IR, 1H NMR spectral methods. Absorption spectroscopy, emission, viscosity measurements, DNA melting and DNA photocleavage and the binding of the two complexes with calf thymus DNA have investigated. The spectroscopic studies together with viscosity measurements and DNA melting studies support that both of these complexes bind to CT DNA by groove mode, (1) binds more avidly to CT DNA than (2) which is consistent with the extended planar and p system of 1, 10, phenanthroline. Noticeably, the two complexes have been found to be efficient photosensitisers for strand scissions in plasmid DNA.