The evolution of microstructure and composition of titanium dioxide (TiO2) nanotube arrays fabricated with hydrofluoric acid (HF) and NH4F electrolytes as a function of annealing temperature up to 400 degrees C was investigated and compared using X-ray diffraction, scanning electron microscopy, and X-ray absorption near-edge structure spectroscopy. Results showed that TiO2 nanotube arrays grown in HF electrolyte contained 90% amorphous TiO2 and 10% lower oxidation states of titanium from Ti2+ (TiO) and Ti3+ (Ti2O3) cations. After annealing at 400 degrees C, TiO2 nanotube arrays underwent charge transfer and phase transformation to 93% anatase phase, 6% amorphous TiO2, and 1% suboxides. In contrast, as-grown TiO2 nanotube arrays using NH4F electrolyte possessed less amorphous TiO2 (82%) but more suboxides (18%) due to lower oxygen ion formation from scanty 3 wt % of H2O addition. Its onset temperature of phase transformation was found to be higher than TiO2 nanotube arrays prepared by HF solution. Moreover, when annealed to 400 degrees C, the crystallinity of TiO2 nanotube arrays increased only by 86% for the anatase phase. The lower anatase phase could be attributed to the formation of (NH4)(2)TiF6 type compounds presumably formed by the reaction of TiF62- and NH4+ ions dissociated from NH4F. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3533388] All rights reserved.