In this study, the effects of aging on crystalline phases, crystallite sizes and agglomeration of TiO2 gels were investigated using thermogravimetry (TG), differential thermal analysis (DTA), X-ray diffraction (XRD), BET specific surface area analyzer and transmission electron microscopy (TEM). The activation energies of anatase crystallite growth for aged and unaged gels were calculated to determine and explain the growth mechanism. TiO2 aging treatment decreased the number for hydroxyl ions existing in the anatase, resulting in lowering the rate of surface area reduction and decreasing the amount of agglomeration after calcination. The activation energy values for crystallite growth in samples with aging treatment were greater than that found in unaged samples. In nanocrystalline anatase samples without aging treatment, the oriented attachment controlled the crystallite growth. However, grain boundary diffusion controlled the crystallite growth for nanocrystalline anatase samples with aging treatment. (c) 2005 Elsevier B.V. All rights reserved.