The mole fractions of hydroxo complexes of titanium(IV) ion in an aqueous solution with 0.10 mol dm(-3) NaClO4 at 25degreesC have been determined as a function of pH by a newly developed analytical procedure based on UV spectrophotometry, using a metastable homogeneous solution of 1.25 x 10(-4) mol dm(-3) in total concentration of Ti(IV). Also, the total concentration of the hydroxo complexes in equilibrium with Ti(OH)(4) solid, or the solubility of Ti(OH)(4) solid in an inhomogeneous system, has been obtained by ICP measurement for the solution phase. A combination of these data yielded the absolute concentration of each complex species in equilibrium with Ti(OH)(4) solid. Finally, Ti(OH)(3)(+) complex has been assigned to the precursor for the formation of anatase TiO2 nanoparticles transformed from Ti(OH)(4) gel from a comparison between the above equilibrium data and a kinetic study on the formation rate of the anatase TiO2 particles in the gel-sol system.