Ordered porous niobium oxide with the diameter of less than 10 nm and the aspect ratio of more than 20 is prepared by anodization of niobium. foils at 2.5 V in the mixture of 1 wt% HF and 1 M H3PO4 for 1 h. In this study, the effects of the mixed electrolytes, anodic potential and anodization time on the preparation of porous niobium oxide are described based on the current-time transients during anodization and morphological observations. It is founded that a single HF electrolyte leads to the formation of pores as well as the fast dissolution of formed pores at the surface. The dissolution of the formed oxide is significantly retarded by the addition of appropriate amount of H3PO4. (c) 2006 Elsevier Ltd. All rights reserved.