Atomic force microscopy (AFM) was used to investigate the topographic characteristics of pitting sites during anodic galvanostatic etching of high-purity aluminum foils in hot baths containing HCl and H2SO4. Pit sites near pre-existing ridges formed by rolling are preferred, and the distribution of sites depends on the foil fabrication process. Etch tunnels are found only within shallow pits formed by surface dissolution, which may be a prerequisite for tunnel formation. Pit sites are more numerous and evenly distributed after pretreatment of foils by immersion in 1 N NaOH. AFM imaging during acid dissolution of NaOH treated foils reveal submicrometer cavities formed as a result of NaOH treatment. Some cavities appear to form near oxide particles or inclusions. It is argued that these cavities are subsurface voids exposed by uniform dissolution in the acid. If the void surface is oxide-free, they may dissolve as pits when exposed by the shallow surface dissolution shown to take place in the etchant bath. (C) 2001 The Electrochemical Society. All rights reserved.