The pH value in the gel-sol system for the preparation of uniform anatase TiO2 nanoparticles, as a decisive factor for controlling the size and shape of the final product, was found to be significantly changed during the formation process of the anatase TiO2 particles from a condensed Ti(OH)(4) gel. The dramatic evolution of pH with the progress of the synthetic process has clearly been explained in terms of the adsorption and desorption of a hydroxide ion (OH-) or a proton (H+) on the solids transforming with time. The adsorption and desorption of OH- or H+ were enhanced by the presence of an inert electrolyte such as NaClO4, as explained by its shielding effect on the electrical interactions between the electrically charged precipitates and free OH- and H+ ions. The electrolyte also hampered the phase transformation of Ti(OH)(4) precipitate to anatase TiO2, This effect of electrolytes was explained in terms of the inhibited nucleation of anatase TiO2 by enhanced adsorption of OH- ions to TiO2 embryos. The points of zero charge (PZC) of the amorphous Ti(OH)(4) precipitate and the anatase TiO2 particles at 25degreesC were obtained from the change in pH associated with the adsorption and desorption of OH- or H+, i.e., 4.6 for Ti(OH)(4) precipitate and 6.0 for anatase TiO2 in the presence of 0.1 mol dm(-3) NaClO4. The PZC of the Ti(OH)(4) precipitate measured at 25degreesC after additional aging at 100degreesC for 30 min was shifted to 4.1, owing to the promoted adsorption of OH-.