The stability of complexes of NaDNA with bipyridyl-(ethylenediamine)platinum(II) (abbreviated [(bipy)Pt(en)]²⁺) and with netropsin has been studied using two techniques: (i) ultraviolet (UV) melting experiments were done on NaDNA?[(bipy)Pt(en)]²⁺, showing that the [(bipy)Pt(en)]²⁺ ligand stabilizes the DNA double helix structure; and (ii) swelling measurements (via optical microscopy) as a function of relative humidity were done on wet-spun oriented films of NaDNA?[(bipy)Pt(en)]²⁺ and of NaDNA?netropsin. The swelling data shows that an irreversible transition of the films occurs at high relative humidity, first for the NaDNA?netropsin, then for pure NaDNA, and lastly for the NaDNA?[(bipy)Pt(en)]²⁺. These results are indicative that the [(bipy)Pt(en)]²⁺ complex stabilizes the intermolecular bonds which mediate the film swelling characteristics. A model is suggested for the binding of [(bipy)Pt(en)]²⁺ to DNA to explain why the swelling experiments show this ligand as increasing the intermolecular bond strength between the DNA double helices, while netropsin decreases this degree of stabilization.