The preparation and conditioning of DNA-modified electrodes, in which glassy carbon electrodes are covered by a thick film of double-stranded DNA and subsequently conditioned in acetate buffer, has been followed with Electrochemical Impedance Spectroscopy. Spectra were recorded at different values of applied potential in order to probe the alterations occurring in the modifier layer. The alterations in the EIS response parallel the changes in voltammetric response and show that the modifier layer is a good conductor and that the behaviour as a DNA-biosensor is essentially due to surface phenomena and interaction with single-stranded DNA in solution during conditioning.