A water-splitting reactor yielding hydrogen and oxygen was designed with a titanium oxide (TiO2) nanotube array photoelectrode vertically grown on a titanium substrate. The TiO2 nanotube arrays were made by the method of anodization and annealed in an oxygen atmosphere. Hydrogen gas was collected from the reactor and the exact amount of hydrogen gas evolved from the photoanode was analyzed. The relationship between the amount of hydrogen evolution and three key factors, viz. the tube length, tube structure and crystal structure, was investigated. (C) 2008 Elsevier B.V. All rights reserved.