Molecular and atomic adsorption of hydrogen on anatase TiO2 nanotubes is investigated using density functional theory. Hydrogen molecules are found to physisorb nondissociatively on the Ti atoms, with little effect on the electronic property of the nanotubes. Differently, a semiconductor-to-metal transition occurs when the hydrogen atoms chemisorb only on the twofold-coordinated O atoms. Further increasing hydrogen concentration will first lead to a 'heterolytic adsorption mode' where the adjacent Ti atoms can also adsorb the hydrogen atoms, and finally the full adsorption in both molecular and atomic forms. The observed adsorption-dependent electronic properties of TiO2 nanotubes is useful for their applications in hydrogen sensors. (c) 2009 Elsevier B.V. All rights reserved.