Treatment of TCNQ thin film electrodes derived either from a 9-aminoacridinium(TCNQ)(2) or from TTF-TCNQ with ethidium bromide solution produced hydrophobic surface layers that inhibited ion motion. The passivation process was shown by electrochemical quartz crystal microgravimetry (EQCM) to involve an ion exchange step and a subsequent anodic oxidation coupled to the unidirectional ejection of countercations from the films. The quartz crystal microgravimetry (QCM) frequency of the passivated films exhibited a slow increase which was attributed to loss of water and/or disruption of the hydrogen bonding network at the ethidium-modified interface. The aqueous solution electroactivity of the ethidium-modified films could be restored by exposure of the surfaces to dilute solutions of herring testes DNA, which turned on (or "gated") the square wave EQCM ion motion at these electrodes.