The interest of high frequency impedance measurements performed on decanted solids was demonstrated for the in situ investigation of DNA adsorption onto polypyrrole (PPy). The dielectric method was first applied to distinguish between polymers containing different dopant and to monitor their ion-exchange occurring when PPy was contacting a phosphate buffer solution. Results were compared to those obtained by X-ray photoelectron spectroscopy (XPS) and ion-exchange chromatography. Afterwards, the effects of DNA adsorption were investigated in 0.01 M phosphate buffer at pH 7. A significant decrease in the dielectric parameters was observed upon the adsorption, as a result of the coverage of the conductive PPy by an insulating layer of DNA, while maintaining, however, the mobility of doping anions in the bulk of the polymer. Irreversibility of the adsorption process was assessed. Some conformational changes of the adsorbed DNA have been suspected to occur during the deposition of increasing amounts of DNA.