The interaction with DNA of the platinum(II) square planar complexes [Pt(N-N)(py)(2)](2+) (N-N = 1, 10-phenanthroline (phen), dipyrido[3,2-d-.2',3'- quinoxaline (dpq), dipyrido[3,2-a:2',3'-c]phenazine (dppz), benzodipyrido[b:3,2-h:2'3'-phenazine (bdppz)) has been investigated by means of absorption, circular and linear dichroism spectroscopy, DNA melting, and viscosity. In the presence of excess [DNA] all the complexes intercalate to the double helix. For those with the most extended phenanthrolines the binding mode depends on the [DNA]/[complex] ratio (q); at low q values the substances bind externally to DNA probably self-aggregating along the double helix. When the DNA concentration is large enough, the aggregate breaks up and the complex intercalates within the nucleobases. The complexes self-aggregate, without added DNA, in the presence of a large salt concentration.