Nanometer scale morphological changes in the passive oxide on aluminum have been tracked as a function of polarization in aqueous chloride and borate electrolytes. Nanoscale void formation has been detected and characterized in the passive oxide on single and polycrystalline Al as well as nanocrystalline Al films. Void nucleation occurs at the metal/oxide interface and growth proceeds into the oxide. Nucleation and growth are continuous processes that occur well below the pitting potential. Void growth is related to the rate and extent of the passive oxide growth. Chloride is shown not to be necessary for the nucleation and growth of voids. The extent of void growth correlates with the faradaic charge density produced due to Al oxidation. Void densities on the order of 5 x 10(10) cm(-2) form with as little as 2 to 4 monolayers of Al oxidation and at volumetric efficiencies of 10 to 50%. The mechanistic origin of the voids is consistent with point defect saturation at the Al/oxide interface. The shape factors for the voids are inconsistent with two leading pit initiation models where stable pitting is argued to result from disruption of the remnant oxide over a void or voidlike structures. (c) 2006 The Electrochemical Society.