Hollow, spherical TiO2 microparticles of several tens of micrometers in diameter can be prepared by spraying water into an organic phase containing titanium tetraisopropoxide (TTIP) as a titanium source. The concentration of TTIP did not affect the shell thickness. On the contrary, the shell thickness was increased with the concentration of the additives such as acetic acid and accetylacetone, having effects to protect TTIP from hydrolysis and condensation. The formation of a hollow particle was described by a simple model involving the hydrolysis of TTIP at the water oil interface, the inward diffusion of hydrolyzed titanium hydroxide through the passage in the shell and its incorporation into the TiO2 shell by condensation. The reduction of porosity of shell inhibits the diffusion, resulting in the fortnation of hollow structure. The simulation based on this model predicted that the shell thickness increased as the diffusion rate increased or the reaction rate decreased, and was independent of the outer TTIP concentration. These predictions were in qualitative agreement with the experimental results. (c) 2007 Elsevier B.V. All rights reserved.