Complexes of NaDNA with the bipyridyl-(ethylenediamine)platintum(II) (abbreviated [(bipy)Pt(en)]²⁺) molecular ion have been studied in solution via ultraviolet absorption experiments at 260 nm between 50 and 90 °C. These measurements, performed as a function of the molar ratio of the [(bipy)Pt(en)]²⁺ complex to DNA base pairs, show that the stability of the DNA double helix is increased by the formation of the DNA·[(bipy)Pt(en)]²⁺ complex: at a molar ratio of 0.33, the temperature at which the DNA double helix separates into two single strands is increased by about 15 °C.