A series of new flexible diamine-bridged dinuclear ruthenium(II) complexes [{RuCl(bpy)(2)}(2)(mu-BL)](2+) (bpy = 2,2'-bipyridine; BL = H2N(CH2),NH2 (n = 4 ([1](2+)), 8 ([2](2+)), 12 ([3](2+)))) were prepared. Time-course spectroscopic experiments revealed that these dinuclear ruthenium(II) complexes selectively bind to guanine residues in calf thymus (CT) DNA and the degree of binding of [2](2+) or [3](2+) to CT-DNA relative to [1](2+) is high.