New experimental results concerning the adsorption and anodic oxidation of DNA denatured in acid at glassy carbon electrodes using differential pulse, square wave and cyclic voltammetry, and impedance have been obtained, in the pH range 0-12 and as a function of adsorption potential and time of adsorption. The assumption up until now of a completely irreversible oxidation of guanine and adenine residues is shown not to be true. An explanation for these and previous results, based on desorption of the oxidation products and their diffusion away from the electrode surface, physically impeded for multilayer adsorption of DNA residues, is proposed. This situation corresponds to long adsorption times, as is the case here, and is corroborated by scanning electron microscopy.