Association constants for ruthenium(III) hexaamine and cobalt(III) tris(2,2'-bipyridine) with solution and surface-immobilized DNA were determined. The interaction of the cationic redox molecules with calf thymus DNA was monitored via normal pulse voltammetry with analysis of the mass-transfer limited current assuming a discrete binding-site model. Single-stranded DNA was immobilized on gold via self-assembly of a 5' hexanethiol linker. Double-stranded surface-immobilized DNA was produced by hybridization of a complementary target to surface-immobilized single strands. The interaction between the metal complexes and surface-immobilized DNA was determined using chronocoulometry to construct adsorption isotherms. The measured binding constants for the cationic redox molecules with solution, surface-immobilized single-stranded, and double-stranded DNA are well-correlated, even as a function of ionic strength. The agreement between the determined association constants for the surface-immobilized and solution DNA indicates the potential utility of DNA-derivatized electrodes for examination of small molecule interactions with nucleic acids.