High-resolution transmission electron microscopy and electron energy loss spectroscopy/energy-dispersive X-ray analysis were employed to reveal the development of nanoporous, amorphous anodic oxide films on niobium. The oxide was formed at 10 A m(-2) in a dehydrated phosphate/glycerol electrolyte at 453 K. The relatively nonuniform pore network comprised pores about 5 nm wide, which connect the film surface to an similar to 5 nm thick barrier layer at the film base. The film is presumed to grow at the metal/oxide interface by inward migration of O2- ions through the barrier layer, while field-assisted dissolution proceeds at the pore bases. (C) 2005 The Electrochemical Society.