The study examines anodic films formed on bulk and sputtered zirconium in ammonium fluoride/glycerol electrolyte with up to 5 vol.% added water, using film cross-sections prepared by focused ion beam and ultramicrotomy. The findings indicate porous films consisting of zirconium fluoride surrounding zirconia-based nanotubes. The fluoride arises due to faster migration of F- ions relative to O2- ions; its dissolution exposes the nanotubes. Ion beam analyses revealed the highest amount of fluorine for films formed in electrolyte with no water addition. O-18 tracer indicated that water was the primary source of oxygen in the films, which grew at an efficiency of similar to 80%. (C) 2011 Elsevier Ltd. All rights reserved.