The cerium ion (Ce4+) modified titania sol and nanocrystallites were prepared by chemical coprecipitation-peptization and hydrothermal synthesis methods, respectively. XRD patterns show that Ce4+-TiO2 sol particles had anatase semicrystalline structure. And the calcined Ce4+-TiO2 powder was composed of predominant anatase titania and crystalline cerium titanate (11.18 wt.% CexTi(1-x)O2). AFM micrograph shows that ultrafine particles were well dispersed in sol system and average particle size was about 10 nm. Ce4+-TiO2 nanocrystallites have grown into 70 nm in mean size. The difference in calculated particle size (2.41 nm for sol particle and 4.53 nm for crystallite) by XRD Scherrer's formula was mainly due to aggregation effect of nanoparticles. The experimental results exhibit that Ce4+-TiO2 sol and nanocrystallites can effectively photodegrade reactive brilliant red dye (X-3B) with the dye/Ce4+-TiO2/visible-light reaction system. Moreover, photocatalytic reaction also can carry out in hydrosol reaction system as well as in suspension reaction system. And Ce4+-TiO2 sol has shown higher efficiency than nanocrystallites in respect of photocatalytic activity. Meanwhile, dye photodegradation mechanisms involving photolysis, photocatalysis, photosensitized photocatalysis and interband photocatalysis were proposed regarding different photocatalytic reaction system. (C) 2003 Elsevier B.V. All rights reserved.