TiO2 colloidal particles with a fixed crystalline phase of anatase or rutile were heat-treated under different conditions (air-heating and hydrothermal-heating) to modify their surface characters which essentially affected the adsorption behavior of a Ru-complex dye on the surfaces. Changes in the number of surface OH group (N-(OH)), apparent crystallite size, pore structure were evaluated as a function of heat-treatment conditions for various TiO2 nanoparticles. The amount of a Ru-complex dye adsorbed (N-(dye)) was correlated to those surface properties of TiO2 samples. N-(dye) values tended to increase with increasing crystallinity for only the anatase-type TiO2 particles. However, there was no simple, common relationship between N-(dye) and N-(OH) for all the TiO2 nanoparticles examined. Considerable difference in N-(dye) values obtained in different nanoparticles might be explained on the basis of the surface pore structures of those particles. (C) 2003 Kluwer Academic Publishers.