The evolution of microstructure and composition of titanium dioxide (Ti O2) nanotube arrays fabricated with hydrofluoric acid (HF) and N H4 F electrolytes as a function of annealing temperature up to 400°C was investigated and compared using X-ray diffraction, scanning electron microscopy, and X-ray absorption near-edge structure spectroscopy. Results showed that Ti O2 nanotube arrays grown in HF electrolyte contained 90% amorphous Ti O2 and 10% lower oxidation states of titanium from Ti2+ (TiO) and Ti3+ (Ti2 O3) cations. After annealing at 400°C, TiO2 nanotube arrays underwent charge transfer and phase transformation to 93% anatase phase, 6% amorphous Ti O 2, and 1% suboxides. In contrast, as-grown Ti O2 nanotube arrays using NH4F electrolyte possessed less amorphous TiO 2 (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°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 (NH 4)2 Ti F6 type compounds presumably formed by the reaction of Ti F6 2- and NH4+ ions dissociated from N H 4 F.