The formation of porous Ta2O5 on tantalum was investigated in H2SO4 electrolytes containing low concentrations of HF (0.1-5 wt %). Under optimized electrochemical conditions, porous Ta2O5 consisting of self-assembled pore arrays with single pore diameters of similar to 20 nm and a pore spacing of similar to 15 nm forms. The pore structure and the pore distribution depend on the concentration of HF, the anodization voltage, and the time for anodic oxidation. Porous layers; 400 nm thick with a regular pore distribution can be formed. For thicker layers cracking and a tendency for delamination was observed. (C) 2005 The Electrochemical Society.