Nanoscopic defects present in ultrathin (similar to 6 nm) silica films covalently attached to gold substrates through a gold oxide layer exhibit a voltammetric response consistent with a random array of ultramicroelectrodes. These pinholes can be passivated via electrochemical polymerization of phenol to create insulating poly (phenylene) oxide plugs as documented by atomic force microscopy and infrared reflectance-absorbance spectroscopy. Passivation of pinholes is similar to 99.5% complete after 550 voltammetric cycles of oxidative electropolymerization. (C) 2009 Elsevier B.V. All rights reserved.