Uniform anatase-type TiO2 nanoparticles were prepared by the gel-sol process from a condensed Ti(OH)(4) gel preformed by the hydrolysis of a Ti-triethanolamine (TEOA) complex. The particle size of the anatase TiO2 was increased front ca. 5 to 30 mu with pH increasing from 0.6 to 12 by aging the Ti(OH)(4) gel at 140degreesC for 72 h, while the yield of the anatase TiO2, 100% below pH 9.6, started to decrease from pH 10, to 67% at pH 11.5 and only 9% at pH 12.2. These results reveal a significant reduction of the nucleation rate of the anatase TiO2 with increasing pH, as is explained by the reduction of the concentration of a precursor complex, Ti(OH)(3)(+), and the adsorption of hydroxide ion onto the embryos of TiO2. Triethanolamine appeared to enhance the pH effect on the nucleation rate of anatase TiO2 particles by adsorption onto their embryos, leading to the wide range of the size control. Triethanolamine was also found to act as a shape controller of the anatase TiO2 particles for yielding ellipsoidal particles from Ti(OH)(4) gel at a relatively high pH above 11. The particle size was also controlled by seeding of anatase TiO2. Moreover, the seeding experiment suggested that the rate-determining step of the gel-sol process was not the dissolution of the hydroxide gel, but the deposition of the monomeric precursor from the solution phase. (C) 2003 Elsevier Science (USA). All rights reserved.