The present work demonstrates that an amorphous anodic oxide on titanium can be converted to a micrometer thick nanoporous rutile layer upon extended polarization in a fluoride-containing electrolyte, whereas extended polarization in a fluoride-free electrolyte leads to a conversion to anatase. Titanium was preanodized in fluoride-free 1 M H3PO4 for 10 min, then anodization was either continued for 5 h in the presence or absence of fluoride ions and the resulting layers characterized. Scanning electron microscopy, X-ray diffraction, and Raman and photocurrent spectroscopy confirm ion-selective structure conversion and extended layer growth at room temperature. (c) 2007 The Electrochemical Society.