Nanometer crystalline titania (TiO2) particles with high photo-catalytic activity can be obtained from pyrolysis of titanyl organic compounds. The particle sizes are mainly affected by calcinating temperature, and then the composition of crystal phase is affected not only by calcination temperature but also by calcination time. TGA-FTIR coupling analysis showed that some Ti4+ ions on the surface of TiO2 were induced into Ti3+ ones (oxygen vacancies) by carbon (C) formed from pyrolysis of titanyl organic compounds, on the other hand, some OH groups were formed on crystal surface for the presence of water from pyrolysis, which was proved on by XPS study. The researches on photo-catalytic activity as well as analysis of XPS indicate that the photo-catalytic activity of TiO2 was related to surface Ti3+ capacity, surface OH and Ti3+/OH ratio. When the Ti3+/OH ratio on the surface was close to 1, the nanometer crystalline TiO2 particles showed better photo-catalytic activity. (c) 2004 Elsevier B.V. All rights reserved.