This study examines the superhydrophilicity and chemical state of boron-doped TiO2 prepared by the ion implantation method and explores the effect of annealing on them. XRD measurements show that the implanted sample forms a polycrystalline structure of anatase with no trace of rutile. Their water-contact angles are significantly reduced upon irradiation with ultraviolet light. The Ti 2p XPS spectra of the oxide exhibit a shoulder peak at the lower binding energy side of the main peak, whereas no shoulder is observed in single-crystal and annealed TiO2. This suggests that the titanium ions with lower valences are produced by boron doping and they disappear on subsequent annealing. The SIMS depth pro. le of boron in the as-implanted TiO2 exhibits a peak at a depth of 15-20 nm, whereas this peak disappears upon annealing. The water-contact angles increased on annealing under ultraviolet light illumination. It is concluded that the improvement in the superhydrophilicity by boron doping is due to the reduction of titanium, and the deterioration of superhydrophilicity with the subsequent annealing is due to the oxidation of reduced titanium and the inward diffusion of boron. (C) 2008 Elsevier B. V. All rights reserved.