We report on the anodic formation of titania nanotubes with a uniform diameter of about 20 nm and an aspect ratio over 1000: 1 in an HClO4 solution. The as-prepared product was obtained as an amorphous structure, which can be crystallized to a pure anatase structure with hollow characteristics remaining. The cross-sectional scanning electron microscopy images of interfaces between the Ti substrate and titania indicate that the gradual structural transformation of titania induced by concentrated ion species via localized electrochemical reactions is the underlying mechanism for the formation of nanotubes. (c) 2008 The Electrochemical Society.