Guanine, guanosine, and adenine have been electrochemically oxidized at a glassy carbon electrode in aqueous solution. Electrochemical impedance is shown to be a valuable probe of adsorption of the oxidation products on the electrode surface. Voltammetric and impedance studies at different concentrations have been carried out. Correlation between the results for the different compounds helps in understanding the adsorption process and the preconcentration properties of purine bases. Irradiation with ultrasound during recording of the impedance spectrum enabled adsorption to be avoided and the electrode process of the solution-soluble species to be investigated, and oxidation rate constants were calculated.