The present work considers defect disorder of TiO2 and the related electrical properties, including electrical conductivity and thermoelectric power, at elevated temperatures. Defect chemistry of TiO2 is described in terms of the relationships between the concentration of defects and oxygen activity. These relationships may be used for the imposition of controlled defect disorder and the related functional properties of TiO2, including electronic structure and charge transport. It is shown that defect chemistry may be used as a framework for tailoring the photoelectrochemical properties of TiO2 in order to achieve those properties required for optimised photoelectrode performance in solarhydrogen generation. Defect chemistry may also be used for the imposition of surface active sites responsible for enhanced photoreactivity between the TiO2 surface and water. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.