Gold electrodes were modified with DNA by adsorption. The DNA-modified electrodes were electrochemically characterized with Co(bpy)(3+)(3), a electroactive DNA-binding complex, as an indicator. It is interesting that the pair of redox peaks of Co(bpy)(3+)(3) split into two pairs at dsDNA-modified gold electrodes. One pair of peaks shifts negative, and the peak current increases notably; another pair of peaks shifts positive. These suggest that dsDNA has been immobilized onto gold electrode surfaces and the layer of dsDNA on the surfaces can bind Co(bpy)(3+)(3) in two different ways. Gold electrodes can be modified also with ssDNA by adsorption but only one pair of peaks of Co(bpy)(3+)(3) appears at ssDNA-modified gold electrodes. The amount of Co(bpy)(3+)(3) enriched by the layers of dsDNA or ssDNA adsorbed at gold electrodes was estimated from the peak charge of Co(bpy)(3+)(3) reduction at the electrodes obtained by CV. The stability of the DNA-modified electrodes was investigated. The DNA modification layer on gold surfaces is unstable to alkali and to heat, but stable to acid solutions and very stable in long stock in a dry state. A comparison of modifications of gold, platinum and glassy carbon with DNA was carried out.