The cathodic reactions of electrolytic nanocrystalline TiO2 coating on pure titanium were investigated by polarization tests in TiCl4 alcoholic and HCl aqueous solutions, respectively. Three major reactions which belong to various applied voltage ranges were identified: (i) O-2 + 4H(+) + 4e(-) --> 2H(2)O, from -0.1 to -0.3 V (Ag/AgCl), (ii) 2H(+) + 2e(-) --> H-2, from -0.3 to -0.9 V, and (iii) 2H(2)O + 2e(-) --> H-2 + 2OH(-), from -0.9 to -2.5 V. Excellent deposition was located at the third stage, because H+ ions were depleted and many more OH- ions were provided to form TiO(OH)(2).H2O on the cathodic surface. The coated films were further characterized by thermogravimetric differential thermal analysis, X-ray diffraction, X-ray photoelectron spectrometer, scanning electron microscopy, atomic force microscopy, dynamic cyclic polarization, and cell bioactivity assay. After being annealed at 300degreesC for 1 h, the coated films were further condensed into anatase TiO2, and gradually transformed into rutile TiO2 at 500degreesC. From cross section, the TiO2 coated film similar to1.2 mum thick revealed excellent adhesion with substrate. Compared with thermally grown oxide TiO2 film, much better bioactivity and more corrosion resistance were found on the electrolytic TiO2 coated specimens. (C) 2004 The Electrochemical Society.