We have found that the mixed-ligand complex Pt(trpy)OH+, where trpy denotes 2,2’:6’,2"-terpyridine, binds to DNA by competitive covalent and intercalative binding modes. Intercalation predominates in the kinetically controlled phase of the reaction, but after extended incubation the complex shifts essentially quantitatively to a covalently bound form. The intercalated form of the complex can exhibit dramatically enhanced photoluminescence except when the complex binds near a guanine residue which quenches the emission by means of an electron-transfer process. We have characterized the binding interactions by a variety of physical methods including absorbance, circular dichroic, and emission spectroscopies as well as viscometry. In addition, we have used DNA from salmon testis, Micrococcus lysodeikticus, and Clostridium perfringens as well as the synthetic forms poly(dA-dT). poly(dA-dT), poly(dG-dC). poly(dG-dC), and poly(dI-dC). poly(dI-dC). For-comparison, we have also carried out a number of parallel investigations with a well-characterized analogue, Pt(trpy)HET(+), where HET denotes 2-hydroxyethanethiol. The results are significant in several different respects. In the first place, this work establishes that platinum(II) terpyridines can be useful reporter probes because the photophysical properties vary dramatically with the local microenvironment. The Pt(trpy)OH+ system is also interesting in that it is a good example of a bifunctional binding agent because the heteroaromatic ligand encourages intercalation while the hydroxide ligand acts as a good leaving group and ensures the possibility of covalent adduct formation. Finally, the results demonstrate how important the interplay between kinetics and thermodynamics can be during the evolution of the interaction of this type of reagent with DNA.