The formation of porous TiO2 on titanium was investigated in H2SO4 electrolytes containing low concentrations of HF (0.05-0.4 wt %). Under optimized electrolyte conditions and extended polarization, highly ordered porous TiO2 is obtained that consists of pore arrays with single pore diameter of 140 nm and a pore spacing of 150 nm. During the formation process, significant current oscillations are observed with an amplitude which depends strongly on the HF content of the electrolyte. Electrochemical, scanning electron microscope and X-ray photoelectron spectroscopy investigations show that the porous layer forms under a competition of TiO2 formation and oxide dissolution up to a limiting thickness of similar to500 nm, and that the time scale for complete self-ordering for the investigated systems is in the order of several hours. (C) 2003 The Electrochemical Society.