A series of mixed-ligand ruthenium(II) complexes of the type [Ru(en)(2)bpy](2+) (bpy=2,2-bipyridine; 1), [Ru(en)(2)phen](2+) (phen=1,10-phenantroline; 2), [Ru(en)(2)IP](2+) (IP=imidazo[4,5-f][1,10]phenanthroline; 3), and [Ru(en)(2)PIP](2+) (PIP=2-phenylimidazo[4,5-f][1,10]phenanthroline; 4) have been isolated and characterized by UV/VIS, IR, and (1)H-NMR spectral methods. The binding of the complexes with calf thymus DNA has been investigated by absorption, emission spectroscopy, viscosity measurements, DNA melting, and DNA photo-cleavage. The spectroscopic studies together with viscosity measurements and DNA melting studies support that complexes 1 and 2 bind to CT DNA (=calf thymus DNA) by groove mode. Complex 2 binds more avidly to CT DNA than complex 1, complexes 3 and 4 bind to CT DNA by intercalation mode, 4 binds more avidly to CT DNA than 3. Noticeably, the four complexes have been found to be efficient photosensitisers for strand scissions in plasmid DNA.