The electrochemical behaviour of TiO2 under X-ray irradiation was studied. Under X-ray irradiation a current, a negative shift in the rest potential, and electrochemical oxidation and decomposition of [Fe-II(CN)(6)](4-) were clearly observed. The incident photon-current conversion efficiency and energy conversion efficiency was 400-2000% and 0.2-2%, respectively, depending on sample conditions. These results show that the photoelectrochemical reactions were promoted by X-rays with a high incident photon-current conversion efficiency. The photocurrent and photopotential were observed above 4.965 keV, which corresponds to the Ti-K edge, indicating that electron-hole pairs are formed during the relaxation process of the excited Ti atoms. (c) 2007 Elsevier Ltd. All rights reserved.